Secure Programming for Linux and Unix HOWTO David A. Wheeler Copyright © 1999, 2000, 2001 by David A. Wheeler This book provides a set of design and implementation guidelines for writing secure programs for Linux and Unix systems. Such programs include application programs used as viewers of remote data, web applications (including CGI scripts), network servers, and setuid/setgid programs. Specific guidelines for C, C++, Java, Perl, Python, TCL, and Ada95 are included. This book is Copyright (C) 1999-2000 David A. Wheeler. Permission is granted to copy, distribute and/or modify this book under the terms of the GNU Free Documentation License (GFDL), Version 1.1 or any later version published by the Free Software Foundation; with the invariant sections being ``About the Author'', with no Front-Cover Texts, and no Back-Cover texts. A copy of the license is included in the section entitled "GNU Free Documentation License". This book is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. ----------------------------------------------------------------------------- Table of Contents 1. Introduction 2. Background 2.1. History of Unix, Linux, and Open Source / Free Software 2.1.1. Unix 2.1.2. Free Software Foundation 2.1.3. Linux 2.1.4. Open Source / Free Software 2.1.5. Comparing Linux and Unix 2.2. Security Principles 2.3. Is Open Source Good for Security? 2.4. Types of Secure Programs 2.5. Paranoia is a Virtue 2.6. Why Did I Write This Document? 2.7. Sources of Design and Implementation Guidelines 2.8. Other Sources of Security Information 2.9. Document Conventions 3. Summary of Linux and Unix Security Features 3.1. Processes 3.1.1. Process Attributes 3.1.2. POSIX Capabilities 3.1.3. Process Creation and Manipulation 3.2. Files 3.2.1. Filesystem Object Attributes 3.2.2. Creation Time Initial Values 3.2.3. Changing Access Control Attributes 3.2.4. Using Access Control Attributes 3.2.5. Filesystem Hierarchy 3.3. System V IPC 3.4. Sockets and Network Connections 3.5. Signals 3.6. Quotas and Limits 3.7. Dynamically Linked Libraries 3.8. Audit 3.9. PAM 3.10. Specialized Security Extensions for Unix-like Systems 4. Validate All Input 4.1. Command line 4.2. Environment Variables 4.2.1. Some Environment Variables are Dangerous 4.2.2. Environment Variable Storage Format is Dangerous 4.2.3. The Solution - Extract and Erase 4.3. File Descriptors 4.4. File Contents 4.5. Web-Based Application Inputs (Especially CGI Scripts) 4.6. Other Inputs 4.7. Human Language (Locale) Selection 4.7.1. How Locales are Selected 4.7.2. Locale Support Mechanisms 4.7.3. Legal Values 4.7.4. Bottom Line 4.8. Character Encoding 4.8.1. Introduction to Character Encoding 4.8.2. Introduction to UTF-8 4.8.3. UTF-8 Security Issues 4.8.4. UTF-8 Legal Values 4.8.5. UTF-8 Illegal Values 4.8.6. UTF-8 Related Issues 4.9. Prevent Cross-site Malicious Content on Input 4.10. Filter HTML/URIs That May Be Re-presented 4.10.1. Remove or Forbid Some HTML Data 4.10.2. Encoding HTML Data 4.10.3. Validating HTML Data 4.10.4. Validating Hypertext Links (URIs/URLs) 4.10.5. Other HTML tags 4.10.6. Related Issues 4.11. Forbid HTTP GET To Perform Non-Queries 4.12. Limit Valid Input Time and Load Level 5. Avoid Buffer Overflow 5.1. Dangers in C/C++ 5.2. Library Solutions in C/C++ 5.2.1. Standard C Library Solution 5.2.2. Static and Dynamically Allocated Buffers 5.2.3. strlcpy and strlcat 5.2.4. libmib 5.2.5. Libsafe 5.2.6. Other Libraries 5.3. Compilation Solutions in C/C++ 5.4. Other Languages 6. Structure Program Internals and Approach 6.1. Follow Good Software Engineering Principles for Secure Programs 6.2. Secure the Interface 6.3. Minimize Privileges 6.3.1. Minimize the Privileges Granted 6.3.2. Minimize the Time the Privilege Can Be Used 6.3.3. Minimize the Time the Privilege is Active 6.3.4. Minimize the Modules Granted the Privilege 6.3.5. Consider Using FSUID To Limit Privileges 6.3.6. Consider Using Chroot to Minimize Available Files 6.3.7. Consider Minimizing the Accessible Data 6.3.8. Consider Minimizing the Resources Available 6.4. Avoid Creating Setuid/Setgid Scripts 6.5. Configure Safely and Use Safe Defaults 6.6. Load Initialization Values Safely 6.7. Fail Safe 6.8. Avoid Race Conditions 6.8.1. Sequencing (Non-Atomic) Problems 6.8.1.1. Atomic Actions in the Filesystem 6.8.1.2. Temporary Files 6.8.2. Locking 6.8.2.1. Using Files as Locks 6.8.2.2. Other Approaches to Locking 6.9. Trust Only Trustworthy Channels 6.10. Set up a Trusted Path 6.11. Use Internal Consistency-Checking Code 6.12. Self-limit Resources 6.13. Prevent Cross-Site Malicious Content 6.13.1. Explanation of the Problem 6.13.2. Solutions to Cross-Site Malicious Content 6.13.2.1. Identifying Special Characters 6.13.2.2. Filtering 6.13.2.3. Encoding 6.14. Be Careful with Data Types 7. Carefully Call Out to Other Resources 7.1. Call Only Safe Library Routines 7.2. Limit Call-outs to Valid Values 7.3. Call Only Interfaces Intended for Programmers 7.4. Check All System Call Returns 7.5. Avoid Using vfork(2) 7.6. Counter Web Bugs When Retrieving Embedded Content 7.7. Hide Sensitive Information 8. Send Information Back Judiciously 8.1. Minimize Feedback 8.2. Don't Include Comments 8.3. Handle Full/Unresponsive Output 8.4. Control Data Formatting (``Format Strings'') 8.5. Control Character Encoding in Output 8.6. Prevent Include/Configuration File Access 9. Language-Specific Issues 9.1. C/C++ 9.2. Perl 9.3. Python 9.4. Shell Scripting Languages (sh and csh Derivatives) 9.5. Ada 9.6. Java 9.7. TCL 10. Special Topics 10.1. Passwords 10.2. Random Numbers 10.3. Specially Protect Secrets (Passwords and Keys) in User Memory 10.4. Cryptographic Algorithms and Protocols 10.5. Using PAM 10.6. Tools 10.7. Windows CE 10.8. Write Audit Records 10.9. Miscellaneous 11. Conclusion 12. Bibliography A. History B. Acknowledgements C. About the Documentation License D. GNU Free Documentation License E. Endorsements F. About the Author List of Tables 4-1. Legal UTF-8 Sequences 4-2. Illegal UTF-8 initial sequences List of Figures 1-1. Abstract View of a Program ----------------------------------------------------------------------------- Chapter 1. Introduction   A wise man attacks the city of the mighty and pulls down the stronghold in which they trust.   Proverbs 21:22 (NIV) This book describes a set of design and implementation guidelines for writing secure programs on Linux and Unix systems. For purposes of this book, a ``secure program'' is a program that sits on a security boundary, taking input from a source that does not have the same access rights as the program. Such programs include application programs used as viewers of remote data, web applications (including CGI scripts), network servers, and setuid/setgid programs. This book does not address modifying the operating system kernel itself, although many of the principles discussed here do apply. These guidelines were developed as a survey of ``lessons learned'' from various sources on how to create such programs (along with additional observations by the author), reorganized into a set of larger principles. This book includes specific guidance for a number of languages, including C, C++, Java, Perl, Python, TCL, and Ada95. This book does not cover assurance measures, software engineering processes, and quality assurance approaches, which are important but widely discussed elsewhere. Such measures include testing, peer review, configuration management, and formal methods. Documents specifically identifying sets of development assurance measures for security issues include the Common Criteria [CC 1999] and the System Security Engineering Capability Maturity Model [SSE-CMM 1999]. More general sets of software engineering methods or processes are defined in documents such as the Software Engineering Institute's Capability Maturity Model for Software (SE-CMM), ISO 9000 (along with ISO 9001 and ISO 9001-3), and ISO 12207. This book does not discuss how to configure a system (or network) to be secure in a given environment. This is clearly necessary for secure use of a given program, but a great many other documents discuss secure configurations. An excellent general book on configuring Unix-like systems to be secure is Garfinkel [1996]. Other books for securing Unix-like systems include Anonymous [1998]. You can also find information on configuring Unix-like systems at web sites such as [http://www.unixtools.com/ security.html] http://www.unixtools.com/security.html. Information on configuring a Linux system to be secure is available in a wide variety of documents including Fenzi [1999], Seifried [1999], Wreski [1998], Swan [2001], and Anonymous [1999]. For Linux systems (and eventually other Unix-like systems), you may want to examine the Bastille Hardening System, which attempts to ``harden'' or ``tighten'' the Linux operating system. You can learn more about Bastille at [http://www.bastille-linux.org] http:// www.bastille-linux.org; it is available for free under the General Public License (GPL). This book assumes that the reader understands computer security issues in general, the general security model of Unix-like systems, and the C programming language. This book does include some information about the Linux and Unix programming model for security. This book covers all Unix-like systems, including Linux and the various strains of Unix, and it particularly stresses Linux and provides details about Linux specifically. There are several reasons for this, but a simple reason is popularity. According to a 1999 survey by IDC, significantly more servers (counting both Internet and intranet servers) were installed in 1999 with Linux than with all Unix operating system types combined (25% for Linux versus 15% for all Unix system types combined; note that Windows NT came in with 38% compared to the 40% of all Unix-like servers) [Shankland 2000]. A survey by Zoebelein in April 1999 found that, of the total number of servers deployed on the Internet in 1999 (running at least ftp, news, or http (WWW)), the majority were running Linux (28.5%), with others trailing (24.4% for all Windows 95/98/NT combined, 17.7% for Solaris or SunOS, 15% for the BSD family, and 5.3% for IRIX). Advocates will notice that the majority of servers on the Internet (around 66%) were running Unix-like systems, while only around 24% ran a Microsoft Windows variant. Finally, the original version of this book only discussed Linux, so although its scope has expanded, the Linux information is still noticeably dominant. If you know relevant information not already included here, please let me know. You can find the master copy of this book at [http://www.dwheeler.com/ secure-programs] http://www.dwheeler.com/secure-programs. This book is also part of the Linux Documentation Project (LDP) at [http://www.linuxdoc.org] http://www.linuxdoc.org It's also mirrored in several other places. Please note that these mirrors, including the LDP copy and/or the copy in your distribution, may be older than the master copy. I'd like to hear comments on this book, but please do not send comments until you've checked to make sure that your comment is valid for the latest version. This book is copyright (C) 1999-2001 David A. Wheeler and is covered by the GNU Free Documentation License (GFDL); see Appendix C and Appendix D for more information. Chapter 2 discusses the background of Unix, Linux, and security. Chapter 3 describes the general Unix and Linux security model, giving an overview of the security attributes and operations of processes, filesystem objects, and so on. This is followed by the meat of this book, a set of design and implementation guidelines for developing applications on Linux and Unix systems. The book ends with conclusions in Chapter 11, followed by a lengthy bibliography and appendices. The design and implementation guidelines are divided into categories which I believe emphasize the programmer's viewpoint. Programs accept inputs, process data, call out to other resources, and produce output, as shown in Figure 1-1 ; notionally all security guidelines fit into one of these categories. I've subdivided ``process data'' into structuring program internals and approach, avoiding buffer overflows (which in some cases can also be considered an input issue), language-specific information, and special topics. The chapters are ordered to make the material easier to follow. Thus, the book chapters giving guidelines discuss validating all input (Chapter 4), avoiding buffer overflows (Chapter 5), structuring program internals and approach (Chapter 6 ), carefully calling out to other resources (Chapter 7), judiciously sending information back (Chapter 8), language-specific information (Chapter 9), and finally information on special topics such as how to acquire random numbers ( Chapter 10). Figure 1-1. Abstract View of a Program A program accepts inputs, processes data,possibly calls out to other programs, and produces output. ----------------------------------------------------------------------------- Chapter 2. Background   I issued an order and a search was made, and it was found that this city has a long history of revolt against kings and has been a place of rebellion and sedition.   Ezra 4:19 (NIV) ----------------------------------------------------------------------------- 2.1. History of Unix, Linux, and Open Source / Free Software 2.1.1. Unix In 1969-1970, Kenneth Thompson, Dennis Ritchie, and others at AT&T Bell Labs began developing a small operating system on a little-used PDP-7. The operating system was soon christened Unix, a pun on an earlier operating system project called MULTICS. In 1972-1973 the system was rewritten in the programming language C, an unusual step that was visionary: due to this decision, Unix was the first widely-used operating system that could switch from and outlive its original hardware. Other innovations were added to Unix as well, in part due to synergies between Bell Labs and the academic community. In 1979, the ``seventh edition'' (V7) version of Unix was released, the grandfather of all extant Unix systems. After this point, the history of Unix becomes somewhat convoluted. The academic community, led by Berkeley, developed a variant called the Berkeley Software Distribution (BSD), while AT&T continued developing Unix under the names ``System III'' and later ``System V''. In the late 1980's through early 1990's the ``wars'' between these two major strains raged. After many years each variant adopted many of the key features of the other. Commercially, System V won the ``standards wars'' (getting most of its interfaces into the formal standards), and most hardware vendors switched to AT&T's System V. However, System V ended up incorporating many BSD innovations, so the resulting system was more a merger of the two branches. The BSD branch did not die, but instead became widely used for research, for PC hardware, and for single-purpose servers (e.g., many web sites use a BSD derivative). The result was many different versions of Unix, all based on the original seventh edition. Most versions of Unix were proprietary and maintained by their respective hardware vendor, for example, Sun Solaris is a variant of System V. Three versions of the BSD branch of Unix ended up as open source: FreeBSD (concentating on ease-of-installation for PC-type hardware), NetBSD (concentrating on many different CPU architectures), and a variant of NetBSD, OpenBSD (concentrating on security). More general information about Unix history can be found at [http://www.datametrics.com/tech/unix/uxhistry/ brf-hist.htm] http://www.datametrics.com/tech/unix/uxhistry/brf-hist.htm and [http://perso.wanadoo.fr/levenez/unix] http://perso.wanadoo.fr/levenez/unix. Much more information about the BSD history can be found in [McKusick 1999] and [ftp://ftp.freebsd.org/pub/FreeBSD/FreeBSD-current/src/share/misc/ bsd-family-tree] ftp://ftp.freebsd.org/pub/FreeBSD/FreeBSD-current/src/share/ misc/bsd-family-tree. Those interested in reading an advocacy piece that presents arguments for using Unix-like systems should see [http://www.unix-vs-nt.org] http:// www.unix-vs-nt.org. ----------------------------------------------------------------------------- 2.1.2. Free Software Foundation In 1984 Richard Stallman's Free Software Foundation (FSF) began the GNU project, a project to create a free version of the Unix operating system. By free, Stallman meant software that could be freely used, read, modified, and redistributed. The FSF successfully built a vast number of useful components, including a C compiler (gcc), an impressive text editor (emacs), and a host of fundamental tools. However, in the 1990's the FSF was having trouble developing the operating system kernel [FSF 1998]; without a kernel the rest of their software would not work. ----------------------------------------------------------------------------- 2.1.3. Linux In 1991 Linus Torvalds began developing an operating system kernel, which he named ``Linux'' [Torvalds 1999]. This kernel could be combined with the FSF material and other components (in particular some of the BSD components and MIT's X-windows software) to produce a freely-modifiable and very useful operating system. This book will term the kernel itself the ``Linux kernel'' and an entire combination as ``Linux''. Note that many use the term ``GNU/ Linux'' instead for this combination. In the Linux community, different organizations have combined the available components differently. Each combination is called a ``distribution'', and the organizations that develop distributions are called ``distributors''. Common distributions include Red Hat, Mandrake, SuSE, Caldera, Corel, and Debian. There are differences between the various distributions, but all distributions are based on the same foundation: the Linux kernel and the GNU glibc libraries. Since both are covered by ``copyleft'' style licenses, changes to these foundations generally must be made available to all, a unifying force between the Linux distributions at their foundation that does not exist between the BSD and AT&T-derived Unix systems. This book is not specific to any Linux distribution; when it discusses Linux it presumes Linux kernel version 2.2 or greater and the C library glibc 2.1 or greater, valid assumptions for essentially all current major Linux distributions. ----------------------------------------------------------------------------- 2.1.4. Open Source / Free Software Increased interest in software that is freely shared has made it increasingly necessary to define and explain it. A widely used term is ``open source software'', which is further defined in [OSI 1999]. Eric Raymond [1997, 1998] wrote several seminal articles examining its various development processes. Another widely-used term is ``free software'', where the ``free'' is short for ``freedom'': the usual explanation is ``free speech, not free beer.'' Neither phrase is perfect. The term ``free software'' is often confused with programs whose executables are given away at no charge, but whose source code cannot be viewed, modified, or redistributed. Conversely, the term ``open source'' is sometime (ab)used to mean software whose source code is visible, but for which there are limitations on use, modification, or redistribution. This book uses the term ``open source'' for its usual meaning, that is, software which has its source code freely available for use, viewing, modification, and redistribution; a more detailed definition is contained in the [http://www.opensource.org/osd.html] Open Source Definition. In some cases, a difference in motive is suggested; those preferring the term ``free software'' wish to strongly emphasize the need for freedom, while those using the term may have other motives (e.g., higher reliability) or simply wish to appear less strident. For information on this definition of free software, and the motivations behind it, can be found at [http://www.fsf.org] http:// www.fsf.org. Those interested in reading advocacy pieces for open source software and free software should see [http://www.opensource.org] http://www.opensource.org and [http://www.fsf.org] http://www.fsf.org. There are other documents which examine such software, for example, Miller [1995] found that the open source software were noticeably more reliable than proprietary software (using their measurement technique, which measured resistance to crashing due to random input). ----------------------------------------------------------------------------- 2.1.5. Comparing Linux and Unix This book uses the term ``Unix-like'' to describe systems intentionally like Unix. In particular, the term ``Unix-like'' includes all major Unix variants and Linux distributions. Note that many people simply use the term ``Unix'' to describe these systems instead. Linux is not derived from Unix source code, but its interfaces are intentionally like Unix. Therefore, Unix lessons learned generally apply to both, including information on security. Most of the information in this book applies to any Unix-like system. Linux-specific information has been intentionally added to enable those using Linux to take advantage of Linux's capabilities. Unix-like systems share a number of security mechanisms, though there are subtle differences and not all systems have all mechanisms available. All include user and group ids (uids and gids) for each process and a filesystem with read, write, and execute permissions (for user, group, and other). See Thompson [1974] and Bach [1986] for general information on Unix systems, including their basic security mechanisms. Chapter 3 summarizes key security features of Unix and Linux. ----------------------------------------------------------------------------- 2.2. Security Principles There are many general security principles which you should be familiar with; one good place for general information on information security is the Information Assurance Technical Framework (IATF) [NSA 2000]. NIST has identified high-level ``generally accepted principles and practices'' [Swanson 1996]. You could also look at a general textbook on computer security, such as [Pfleeger 1997]. A few security principles are summarized here. Often computer security goals are described in terms of three overall goals:   * Confidentiality (also known as secrecy), meaning that the computing system's assets are accessible only by authorized parties.   * Integrity, meaning that the assets can only be modified by authorized parties in authorized ways.   * Availability, meaning that the assets are accessible to the authorized parties in a timely manner (as determined by the systems requirements). The failure to meet this goal is called a denial of service. Some people define additional security goals, while others lump those additional goals as special cases of these three goals. For example, some separately identify non-repudiation as a goal; this is the ability to ``prove'' that a sender sent or receiver received a message, even if the sender or receiver wishes to deny it later. Privacy is sometimes addressed separately from confidentiality; some define this as protecting the confidentiality of a user (e.g., their identity) instead of the data. Most goals require identification and authentication, which is sometimes listed as a separate goal. Often auditing (also called accountability) is identified as a desirable security goal. Sometimes ``access control'' and ``authenticity'' are listed separately as well. In any case, it is important to identify your program's overall security goals, no matter how you group those goals together, so that you'll know when you've met them. Sometimes these goals are a response to a known set of threats, and sometimes some of these goals are required by law. For example, for U.S. banks and other financial institutions, there's a new privacy law called the ``Gramm-Leach-Bliley'' (GLB) Act. This law mandates disclosure of personal information shared and means of securing that data, requires disclosure of personal information that will be shared with third parties, and directs institutions to give customers a chance to opt out of data sharing. [Jones 2000] There is sometimes conflict between security and some other general system/ software engineering principles. Security can sometimes interfere with ``ease of use'', for example, installing a secure configuration may take more effort than a ``trivial'' installation that works but is insecure. OFten, this apparant conflict can be resolved, for example, by re-thinking a problem it's often possible to make a secure system also easy to use. There's also sometimes a conflict between security and abstraction (information hiding); for example, some high-level library routines may be implemented securely or not, but their specifications won't tell you. In the end, if your application must be secure, you must do things yourself if you can't be sure otherwise - yes, the library should be fixed, but it's your users who will be hurt by your poor choice of library routines. A good general security principle is ``defense in depth''; you should have numerous defense mechanisms (``layers'') in place, designed so that an attacker has to defeat multiple mechanisms to perform a successful attack. ----------------------------------------------------------------------------- 2.3. Is Open Source Good for Security? There's been a lot of debate by security practitioners about the impact of open source approaches on security. One of the key issues is that open source exposes the source code to examination by everyone, both the attackers and defenders, and reasonable people disagree about the ultimate impact of this situation. Here are a few quotes from people who've examined the topic. Bruce Schneier argues that smart engineers should ``demand open source code for anything related to security'' [Schneier 1999], and he also discusses some of the preconditions which must be met to make open source software secure. Vincent Rijmen, a developer of the winning Advanced Encryption Standard (AES) encryption algorithm, believes that the open source nature of Linux provides a superior vehicle to making security vulnerabilities easier to spot and fix, ``Not only because more people can look at it, but, more importantly, because the model forces people to write more clear code, and to adhere to standards. This in turn facilitates security review'' [Rijmen 2000]. Elias Levy (Aleph1) discusses some of the problems in making open source software secure in his article "Is Open Source Really More Secure than Closed?". His summary is: So does all this mean Open Source Software is no better than closed source software when it comes to security vulnerabilities? No. Open Source Software certainly does have the potential to be more secure than its closed source counterpart. But make no mistake, simply being open source is no guarantee of security. John Viega's article [http://dev-opensourceit.earthweb.com/news/ 000526_security.html] "The Myth of Open Source Security" also discusses issues, and summarizes things this way: Open source software projects can be more secure than closed source projects. However, the very things that can make open source programs secure -- the availability of the source code, and the fact that large numbers of users are available to look for and fix security holes -- can also lull people into a false sense of security. [http://www.linuxworld.com/linuxworld/lw-1998-11/lw-11-ramparts.html] Michael H. Warfield's "Musings on open source security" is much more positive about the impact of open source software on security. Fred Schneider doesn't believe that open source helps security, saying ``there is no reason to believe that the many eyes inspecting (open) source code would be successful in identifying bugs that allow system security to be compromised'' and claiming that ``bugs in the code are not the dominant means of attack'' [Schneider 2000]. He also claims that open source rules out control of the construction process, though in practice there is such control - all major open source programs have one or a few official versions with ``owners'' with reputations at stake. Peter G. Neumann discusses ``open-box'' software (in which source code is available, possibly only under certain conditions), saying ``Will open-box software really improve system security? My answer is not by itself, although the potential is considerable'' [Neumann 2000]. [http://www-106.ibm.com/developerworks/linux/library/l-oss.html?open&I=252,t= gr,p=SeclmpOS] Natalie Walker Whitlock's IBM DeveloperWorks article discusses the pros and cons as well. Sometimes it's noted that a vulnerability that exists but is unknown can't be exploited, so the system ``practically secure.'' In theory this is true, but the problem is that once someone finds the vulnerability, the finder may just exploit the vulnerability instead of helping to fix it. Having unknown vulnerabilities doesn't really make the vulnerabilities go away; it simply means that the vulnerabilities are a time bomb, with no way to know when they'll be exploited. Fundamentally, the problem of someone exploiting a vulnerability they discover is a problem for both open and closed source systems. It's been argued that a system without source code is more secure in this sense because, since there's less information available for an attacker, it would be harder for an attacker to find the vulnerabilities. A counter-argument is that attackers generally don't need source code, and if they want to use source code they can use disassemblers to re-create the source code of the product. See Flake [2001] for one discussion of how closed code can still be examined for security vulnerabilities (e.g., using disassemblers). In contrast, defenders won't usually look for problems if they don't have the source code, so not having the source code puts defenders at a disadvantage compared to attackers. It's sometimes argued that open source programs, because there's no enforced control by a single company, permit people to insert Trojan Horses and other malicious code. This is true, but it's true for closed source programs - a disgruntled or bribed employee can insert malicious code, and in many organizations it's even less likely to be found (since no one outside the organization can review the code, and few companies review their code internally). And the notion that a closed-source company can be sued later has little evidence; nearly all licenses disclaim all warranties, and courts have generally not held software development companies liable. Borland's Interbase server is an interesting case in point. Some time between 1992 and 1994, Borland inserted an intentional ``back door'' into their database server, ``Interbase''. This back door allowed any local or remote user to manipulate any database object and install arbitrary programs, and in some cases could lead to controlling the machine as ``root''. This vulnerability stayed in the product for at least 6 years - no one else could review the product, and Borland had no incentive to remove the vulnerability. Then Borland released its source code on July 2000. The "Firebird" project began working with the source code, and uncovered this serious security problem with InterBase in December 2000. By January 2001 the CERT announced the existence of this back door as CERT advisory CA-2001-01. What's discouraging is that the backdoor can be easily found simply by looking at an ASCII dump of the program (a common cracker trick). Once this problem was found by open source developers reviewing the code, it was patched quickly. You could argue that, by keeping the password unknown, the program stayed safe, and that opening the source made the program less secure. I think this is nonsense, since ASCII dumps are trivial to do and well-known as a standard attack technique, and not all attackers have sudden urges to announce vulnerabilities - in fact, there's no way to be certain that this vulnerability has not been exploited many times. It's clear that after the source was opened, the source code was reviewed over time, and the vulnerabilities found and fixed. One way to characterize this is to say that the original code was vulnerable, its vulnerabilites because easier to exploit when it was first made open source, and then finally these vulnerabilities were fixed. So, what's the bottom line? I personally believe that when a program is first made open source, it often starts less secure for any users (through exposure of vulnerabilities), and over time (say a few years) it has the potential to be much more secure than a closed program. Just making a program open source doesn't suddenly make a program secure, and making an open source program secure is not guaranteed:   * First, people have to actually review the code. This is one of the key points of debate - will people really review code in an open source project? All sorts of factors can reduce the amount of review: being a niche or rarely-used product (where there are few potential reviewers), having few developers, and use of a rarely-used computer language. One factor that can particularly reduce review likelihood is not actually being open source. Some vendors like to posture their ``disclosed source'' (also called ``source available'') programs as being open source, but since the program owner has extensive exclusive rights, others will have far less incentive to work ``for free'' for the owner on the code. Even open source licenses which have unusually asymmetric rights (such as the MPL) have this problem. After all, people are less likely to voluntarily participate if someone else will have rights to their results that they don't have (as Bruce Perens says, ``who wants to be someone else's unpaid employee?''). In particular, since the most incentivized reviewers tend to be people trying to modify the program, this disincentive to participate reduces the number of ``eyeballs''. Elias Levy made this mistake in his article about open source security; his examples of software that had been broken into (e.g., TIS's Gauntlet) were not, at the time, open source.   * Second, the people developing and reviewing the code must know how to write secure programs. Hopefully the existence of this book will help. Clearly, it doesn't matter if there are ``many eyeballs'' if none of the eyeballs know what to look for.   * Third, once found, these problems need to be fixed quickly and their fixes distributed. Open source systems tend to fix the problems quickly, but the distribution is not always smooth. For example, the OpenBSD developers do an excellent job of reviewing code for security flaws - but they don't always report the identified problems back to the original developer. Thus, it's quite possible for there to be a fixed version in one system, but for the flaw to remain in another. Another advantage of open source is that, if you find a problem, you can fix it immediately. In short, the effect on security of open source software is still a major debate in the security community, though a large number of prominent experts believe that it has great potential to be more secure. ----------------------------------------------------------------------------- 2.4. Types of Secure Programs Many different types of programs may need to be secure programs (as the term is defined in this book). Some common types are:   * Application programs used as viewers of remote data. Programs used as viewers (such as word processors or file format viewers) are often asked to view data sent remotely by an untrusted user (this request may be automatically invoked by a web browser). Clearly, the untrusted user's input should not be allowed to cause the application to run arbitrary programs. It's usually unwise to support initialization macros (run when the data is displayed); if you must, then you must create a secure sandbox (a complex and error-prone task). Be careful of issues such as buffer overflow, discussed in Chapter 5, which might allow an untrusted user to force the viewer to run an arbitrary program.   * Application programs used by the administrator (root). Such programs shouldn't trust information that can be controlled by non-administrators.   * Local servers (also called daemons).   * Network-accessible servers (sometimes called network daemons).   * Web-based applications (including CGI scripts). These are a special case of network-accessible servers, but they're so common they deserve their own category. Such programs are invoked indirectly via a web server, which filters out some attacks but nevertheless leaves many attacks that must be withstood.   * Applets (i.e., programs downloaded to the client for automatic execution). This is something Java is especially famous for, though other languages (such as Python) support mobile code as well. There are several security viewpoints here; the implementor of the applet infrastructure on the client side has to make sure that the only operations allowed are ``safe'' ones, and the writer of an applet has to deal with the problem of hostile hosts (in other words, you can't normally trust the client). There is some research attempting to deal with running applets on hostile hosts, but frankly I'm sceptical of the value of these approaches and this subject is exotic enough that I don't cover it further here.   * setuid/setgid programs. These programs are invoked by a local user and, when executed, are immediately granted the privileges of the program's owner and/or owner's group. In many ways these are the hardest programs to secure, because so many of their inputs are under the control of the untrusted user and some of those inputs are not obvious. This book merges the issues of these different types of program into a single set. The disadvantage of this approach is that some of the issues identified here don't apply to all types of programs. In particular, setuid/setgid programs have many surprising inputs and several of the guidelines here only apply to them. However, things are not so clear-cut, because a particular program may cut across these boundaries (e.g., a CGI script may be setuid or setgid, or be configured in a way that has the same effect), and some programs are divided into several executables each of which can be considered a different ``type'' of program. The advantage of considering all of these program types together is that we can consider all issues without trying to apply an inappropriate category to a program. As will be seen, many of the principles apply to all programs that need to be secured. There is a slight bias in this book towards programs written in C, with some notes on other languages such as C++, Perl, Python, Ada95, and Java. This is because C is the most common language for implementing secure programs on Unix-like systems (other than CGI scripts, which tend to use Perl), and most other languages' implementations call the C library. This is not to imply that C is somehow the ``best'' language for this purpose, and most of the principles described here apply regardless of the programming language used. ----------------------------------------------------------------------------- 2.5. Paranoia is a Virtue The primary difficulty in writing secure programs is that writing them requires a different mindset, in short, a paranoid mindset. The reason is that the impact of errors (also called defects or bugs) can be profoundly different. Normal non-secure programs have many errors. While these errors are undesirable, these errors usually involve rare or unlikely situations, and if a user should stumble upon one they will try to avoid using the tool that way in the future. In secure programs, the situation is reversed. Certain users will intentionally search out and cause rare or unlikely situations, in the hope that such attacks will give them unwarranted privileges. As a result, when writing secure programs, paranoia is a virtue. ----------------------------------------------------------------------------- 2.6. Why Did I Write This Document? One question I've been asked is ``why did you write this book''? Here's my answer: Over the last several years I've noticed that many developers for Linux and Unix seem to keep falling into the same security pitfalls, again and again. Auditors were slowly catching problems, but it would have been better if the problems weren't put into the code in the first place. I believe that part of the problem was that there wasn't a single, obvious place where developers could go and get information on how to avoid known pitfalls. The information was publicly available, but it was often hard to find, out-of-date, incomplete, or had other problems. Most such information didn't particularly discuss Linux at all, even though it was becoming widely used! That leads up to the answer: I developed this book in the hope that future software developers won't repeat past mistakes, resulting in more secure systems. You can see a larger discussion of this at [http:// www.linuxsecurity.com/feature_stories/feature_story-6.html] http:// www.linuxsecurity.com/feature_stories/feature_story-6.html. A related question that could be asked is ``why did you write your own book instead of just referring to other documents''? There are several answers:   * Much of this information was scattered about; placing the critical information in one organized document makes it easier to use.   * Some of this information is not written for the programmer, but is written for an administrator or user.   * Much of the available information emphasizes portable constructs (constructs that work on all Unix-like systems), and failed to discuss Linux at all. It's often best to avoid Linux-unique abilities for portability's sake, but sometimes the Linux-unique abilities can really aid security. Even if non-Linux portability is desired, you may want to support the Linux-unique abilities when running on Linux. And, by emphasizing Linux, I can include references to information that is helpful to someone targeting Linux that is not necessarily true for others. ----------------------------------------------------------------------------- 2.7. Sources of Design and Implementation Guidelines Several documents help describe how to write secure programs (or, alternatively, how to find security problems in existing programs), and were the basis for the guidelines highlighted in the rest of this book. For general-purpose servers and setuid/setgid programs, there are a number of valuable documents (though some are difficult to find without having a reference to them). Matt Bishop [1996, 1997] has developed several extremely valuable papers and presentations on the topic, and in fact he has a web page dedicated to the topic at [http://olympus.cs.ucdavis.edu/~bishop/secprog.html] http:// olympus.cs.ucdavis.edu/~bishop/secprog.html. AUSCERT has released a programming checklist [ftp://ftp.auscert.org.au/pub/auscert/papers/ secure_programming_checklist] [AUSCERT 1996], based in part on chapter 23 of Garfinkel and Spafford's book discussing how to write secure SUID and network programs [http://www.oreilly.com/catalog/puis] [Garfinkel 1996]. [http:// www.sunworld.com/swol-04-1998/swol-04-security.html] Galvin [1998a] described a simple process and checklist for developing secure programs; he later updated the checklist in [http://www.sunworld.com/sunworldonline/swol-08-1998 /swol-08-security.html] Galvin [1998b]. [http://www.pobox.com/~kragen/ security-holes.html] Sitaker [1999] presents a list of issues for the ``Linux security audit'' team to search for. [http://www.homeport.org/~adam/ review.html] Shostack [1999] defines another checklist for reviewing security-sensitive code. The NCSA [http://www.ncsa.uiuc.edu/General/Grid/ACES /security/programming] [NCSA] provides a set of terse but useful secure programming guidelines. Other useful information sources include the Secure Unix Programming FAQ [http://www.whitefang.com/sup/] [Al-Herbish 1999], the Security-Audit's Frequently Asked Questions [http://lsap.org/faq.txt] [Graham 1999], and [http://www.clark.net/pub/mjr/pubs/pdf/] Ranum [1998]. Some recommendations must be taken with caution, for example, the BSD setuid(7) man page [http://www.homeport.org/~adam/setuid.7.html] [Unknown] recommends the use of access(3) without noting the dangerous race conditions that usually accompany it. Wood [1985] has some useful but dated advice in its ``Security for Programmers'' chapter. [http://www.research.att.com/~smb/ talks] Bellovin [1994] includes useful guidelines and some specific examples, such as how to restructure an ftpd implementation to be simpler and more secure. FreeBSD provides some guidelines [http://www.freebsd.org/security/ security.html] FreeBSD [1999] [http://developer.gnome.org/doc/guides/ programming-guidelines/book1.html] [Quintero 1999] is primarily concerned with GNOME programming guidelines, but it includes a section on security considerations. [http://www.fish.com/security/murphy.html] [Venema 1996] provides a detailed discussion (with examples) of some common errors when programming secure prorams (widely-known or predictable passwords, burning yourself with malicious data, secrets in user-accessible data, and depending on other programs). [http://www.fish.com/security/maldata.html] [Sibert 1996] describes threats arising from malicious data. There are many documents giving security guidelines for programs using the Common Gateway Interface (CGI) to interface with the web. These include [http://www.csclub.uwaterloo.ca/u/mlvanbie/cgisec] Van Biesbrouck [1996], [http://language.perl.com/CPAN/doc/FAQs/cgi/perl-cgi-faq.html] Gundavaram [unknown], [http://webreview.com/wr/pub/97/08/08/bookshelf] [Garfinkle 1997] [http://www.eekim.com/pubs/cgibook] Kim [1996], [http://www.go2net.com/people /paulp/cgi-security/safe-cgi.txt] Phillips [1995], [http://www.w3.org/ Security/Faq/www-security-faq.html] Stein [1999], [http://members.home.net/ razvan.peteanu] [Peteanu 2000], and [http://advosys.ca/tips/ web-security.html] [Advosys 2000]. There are many documents specific to a language, which are further discussed in the language-specific sections of this book. For example, the Perl distribution includes [http://www.perl.com/pub/doc/manual/html/pod/ perlsec.html] perlsec(1), which describes how to use Perl more securely. The Secure Internet Programming site at [http://www.cs.princeton.edu/sip] http:// www.cs.princeton.edu/sip is interested in computer security issues in general, but focuses on mobile code systems such as Java, ActiveX, and JavaScript; Ed Felten (one of its principles) co-wrote a book on securing Java ([www.securingjava.com] [McGraw 1999]) which is discussed in Section 9.6 . Sun's security code guidelines provide some guidelines primarily for Java and C; it is available at [http://java.sun.com/security/seccodeguide.html] http://java.sun.com/security/seccodeguide.html. Yoder [1998] contains a collection of patterns to be used when dealing with application security. It's not really a specific set of guidelines, but a set of commonly-used patterns for programming that you may find useful. The Schmoo group maintains a web page linking to information on how to write secure code at [http://www.shmoo.com/securecode] http://www.shmoo.com/ securecode. There are many documents describing the issue from the other direction (i.e., ``how to crack a system''). One example is McClure [1999], and there's countless amounts of material from that vantage point on the Internet. There's also a large body of information on vulnerabilities already identified in existing programs. This can be a useful set of examples of ``what not to do,'' though it takes effort to extract more general guidelines from the large body of specific examples. There are mailing lists that discuss security issues; one of the most well-known is [http:// SecurityFocus.com/forums/bugtraq/faq.html] Bugtraq, which among other things develops a list of vulnerabilities. The CERT Coordination Center (CERT/CC) is a major reporting center for Internet security problems which reports on vulnerabilities. The CERT/CC occasionally produces advisories that provide a description of a serious security problem and its impact, along with instructions on how to obtain a patch or details of a workaround; for more information see [http://www.cert.org] http://www.cert.org. Note that originally the CERT was a small computer emergency response team, but officially ``CERT'' doesn't stand for anything now. The Department of Energy's Computer Incident Advisory Capability (CIAC) also reports on vulnerabilities. These different groups may identify the same vulnerabilities but use different names. To resolve this problem, MITRE supports the Common Vulnerabilities and Exposures (CVE) list which creates a single unique identifier (``name'') for all publicly known vulnerabilities and security exposures identified by others; see [http://www.cve.mitre.org] http:// www.cve.mitre.org. NIST's ICAT is a searchable catalogue of computer vulnerabilities, taking the each CVE vulnerability and categorizing them so they can be searched and compared later; see [http://csrc.nist.gov/icat] http://csrc.nist.gov/icat. This book is a summary of what I believe are the most useful and important guidelines; my goal is a book that a good programmer can just read and then be fairly well prepared to implement a secure program. No single document can really meet this goal, but I believe the attempt is worthwhile. My goal is to strike a balance somewhere between a ``complete list of all possible guidelines'' (that would be unending and unreadable) and the various ``short'' lists available on-line that are nice and short but omit a large number of critical issues. When in doubt, I include the guidance; I believe in that case it's better to make the information available to everyone in this ``one stop shop'' document. The organization presented here is my own (every list has its own, different structure), and some of the guidelines (especially the Linux-unique ones, such as those on capabilities and the fsuid value) are also my own. Reading all of the referenced documents listed above as well is highly recommended. ----------------------------------------------------------------------------- 2.8. Other Sources of Security Information There are a vast number of web sites and mailing lists dedicated to security issues. Here are some other sources of security information:   * [http://www.securityfocus.com] Securityfocus.com has a wealth of general security-related news and information, and hosts a number of security-related mailing lists. See their website for information on how to subscribe and view their archives. A few of the most relevant mailing lists on SecurityFocus are:   + The ``bugtraq'' mailing list is, as noted above, a ``full disclosure moderated mailing list for the detailed discussion and announcement of computer security vulnerabilities: what they are, how to exploit them, and how to fix them.''   + The ``secprog'' mailing list is a moderated mailing list for the discussion of secure software development methodologies and techniques. I specifically monitor this list, and I coordinate with its moderator to ensure that resolutions reached in SECPROG (if I agree with them) are incorporated into this document.   + The ``vuln-dev'' mailing list discusses potential or undeveloped holes.   * IBM's ``developerWorks: Security'' has a library of interesting articles. You can learn more from [http://www.ibm.com/developer/security] http:// www.ibm.com/developer/security.   * For Linux-specific security information, a good source is [http:// www.linuxsecurity.com] LinuxSecurity.com. If you're interested in auditing Linux code, places to see include the Linux Security-Audit Project FAQ and [http://www.lkap.org] Linux Kernel Auditing Project are dedicated to auditing Linux code for security issues. Of course, if you're securing specific systems, you should sign up to their security mailing lists (e.g., Microsoft's, Red Hat's, etc.) so you can be warned of any security updates. ----------------------------------------------------------------------------- 2.9. Document Conventions System manual pages are referenced in the format name(number), where number is the section number of the manual. The pointer value that means ``does not point anywhere'' is called NULL; C compilers will convert the integer 0 to the value NULL in most circumstances where a pointer is needed, but note that nothing in the C standard requires that NULL actually be implemented by a series of all-zero bits. C and C++ treat the character '\0' (ASCII 0) specially, and this value is referred to as NIL in this book (this is usually called ``NUL'', but ``NUL'' and ``NULL'' sound identical). Function and method names always use the correct case, even if that means that some sentences must begin with a lower case letter. I use the term ``Unix-like'' to mean Unix, Linux, or other systems whose underlying models are very similar to Unix; I can't say POSIX, because there are systems such as Windows 2000 that implement portions of POSIX yet have vastly different security models. An attacker is called an ``attacker'', ``cracker'', or ``adversary''. Some journalists use the word ``hacker'' instead of ``attacker''; this book avoids this (mis)use, because many Linux and Unix developers refer to themselves as ``hackers'' in the traditional non-evil sense of the term. That is, to many Linux and Unix developers, the term ``hacker'' continues to mean simply an expert or enthusiast, particularly regarding computers. This book uses the ``new'' or ``logical'' quoting system, instead of the traditional American quoting system: quoted information does not include any trailing punctuation if the punctuation is not part of the material being quoted. While this may cause a minor loss of typographical beauty, the traditional American system causes extraneous characters to be placed inside the quotes. These extraneous characters have no effect on prose but can be disastrous in code or computer commands. I use standard American (not British) spelling; I've yet to meet an English speaker on any continent who has trouble with this. ----------------------------------------------------------------------------- Chapter 3. Summary of Linux and Unix Security Features   Discretion will protect you, and understanding will guard you.   Proverbs 2:11 (NIV) Before discussing guidelines on how to use Linux or Unix security features, it's useful to know what those features are from a programmer's viewpoint. This section briefly describes those features that are widely available on nearly all Unix-like systems. However, note that there is considerable variation between different versions of Unix-like systems, and not all systems have the abilities described here. This chapter also notes some extensions or features specific to Linux; Linux distributions tend to be fairly similar to each other from the point-of-view of programming for security, because they all use essentially the same kernel and C library (and the GPL-based licenses encourage rapid dissemination of any innovations). It also notes some of the security-relevant differences between different Unix implmentations, but please note that this isn't an exhaustive list. This chapter doesn't discuss issues such as implementations of mandatory access control (MAC) which many Unix-like systems do not implement. If you already know what those features are, please feel free to skip this section. Many programming guides skim briefly over the security-relevant portions of Linux or Unix and skip important information. In particular, they often discuss ``how to use'' something in general terms but gloss over the security attributes that affect their use. Conversely, there's a great deal of detailed information in the manual pages about individual functions, but the manual pages sometimes obscure key security issues with detailed discussions on how to use each individual function. This section tries to bridge that gap; it gives an overview of the security mechanisms in Linux that are likely to be used by a programmer, but concentrating specifically on the security ramifications. This section has more depth than the typical programming guides, focusing specifically on security-related matters, and points to references where you can get more details. First, the basics. Linux and Unix are fundamentally divided into two parts: the kernel and ``user space''. Most programs execute in user space (on top of the kernel). Linux supports the concept of ``kernel modules'', which is simply the ability to dynamically load code into the kernel, but note that it still has this fundamental division. Some other systems (such as the HURD) are ``microkernel'' based systems; they have a small kernel with more limited functionality, and a set of ``user'' programs that implement the lower-level functions traditionally implemented by the kernel. Some Unix-like systems have been extensively modified to support strong security, in particular to support U.S. Department of Defense requirements for Mandatory Access Control (level B1 or higher). This version of this book doesn't cover these systems or issues; I hope to expand to that in a future version. More detailed information on some of them is available elsewhere, for example, details on SGI's ``Trusted IRIX/B'' are available in NSA's Final Evaluation Reports (FERs). When users log in, their usernames are mapped to integers marking their ``UID'' (for ``user id'') and the ``GID''s (for ``group id'') that they are a member of. UID 0 is a special privileged user (role) traditionally called ``root''; on most Unix-like systems (including Unix) root can overrule most security checks and is used to administrate the system. Processes are the only ``subjects'' in terms of security (that is, only processes are active objects). Processes can access various data objects, in particular filesystem objects (FSOs), System V Interprocess Communication (IPC) objects, and network ports. Processes can also set signals. Other security-relevant topics include quotas and limits, libraries, auditing, and PAM. The next few subsections detail this. ----------------------------------------------------------------------------- 3.1. Processes In Unix-like systems, user-level activities are implemented by running processes. Most Unix systems support a ``thread'' as a separate concept; threads share memory inside a process, and the system scheduler actually schedules threads. Linux does this differently (and in my opinion uses a better approach): there is no essential difference between a thread and a process. Instead, in Linux, when a process creates another process it can choose what resources are shared (e.g., memory can be shared). The Linux kernel then performs optimizations to get thread-level speeds; see clone(2) for more information. It's worth noting that the Linux kernel developers tend to use the word ``task'', not ``thread'' or ``process'', but the external documentation tends to use the word process (so I'll use the term ``process'' here). When programming a multi-threaded application, it's usually better to use one of the standard thread libraries that hide these differences. Not only does this make threading more portable, but some libraries provide an additional level of indirection, by implementing more than one application-level thread as a single operating system thread; this can provide some improved performance on some systems for some applications. ----------------------------------------------------------------------------- 3.1.1. Process Attributes Here are typical attributes associated with each process in a Unix-like system:   * RUID, RGID - real UID and GID of the user on whose behalf the process is running   * EUID, EGID - effective UID and GID used for privilege checks (except for the filesystem)   * SUID, SGID - Saved UID and GID; used to support switching permissions ``on and off'' as discussed below. Not all Unix-like systems support this.   * supplemental groups - a list of groups (GIDs) in which this user has membership. In the original version 7 Unix, this didn't exist - processes were only a member of one group at a time, and a special command had to be executed to change that group. BSD added support for a list of groups in each process, which is more flexible, and this addition is now widely implemented (including by Linux and Solaris).   * umask - a set of bits determining the default access control settings when a new filesystem object is created; see umask(2).   * scheduling parameters - each process has a scheduling policy, and those with the default policy SCHED_OTHER have the additional parameters nice, priority, and counter. See sched_setscheduler(2) for more information.   * limits - per-process resource limits (see below).   * filesystem root - the process' idea of where the root filesystem ("/") begins; see chroot(2). Here are less-common attributes associated with processes:   * FSUID, FSGID - UID and GID used for filesystem access checks; this is usually equal to the EUID and EGID respectively. This is a Linux-unique attribute.   * capabilities - POSIX capability information; there are actually three sets of capabilities on a process: the effective, inheritable, and permitted capabilities. See below for more information on POSIX capabilities. Linux kernel version 2.2 and greater support this; some other Unix-like systems do too, but it's not as widespread. In Linux, if you really need to know exactly what attributes are associated with each process, the most definitive source is the Linux source code, in particular /usr/include/linux/sched.h's definition of task_struct. The portable way to create new processes it use the fork(2) call. BSD introduced a variant called vfork(2) as an optimization technique. The bottom line with vfork(2) is simple: don't use it if you can avoid it. See Section 7.5 for more information. Linux supports the Linux-unique clone(2) call. This call works like fork(2), but allows specification of which resources should be shared (e.g., memory, file descriptors, etc.). Various BSD systems implement an rfork() system call (originally developed in Plan9); it has different semantics but the same general idea (it also creates a process with tighter control over what is shared). Portable programs shouldn't use these calls directly, if possible; as noted earlier, they should instead rely on threading libraries that use such calls to implement threads. This book is not a full tutorial on writing programs, so I will skip widely-available information handling processes. You can see the documentation for wait(2), exit(2), and so on for more information. ----------------------------------------------------------------------------- 3.1.2. POSIX Capabilities POSIX capabilities are sets of bits that permit splitting of the privileges typically held by root into a larger set of more specific privileges. POSIX capabilities are defined by a draft IEEE standard; they're not unique to Linux but they're not universally supported by other Unix-like systems either. Linux kernel 2.0 did not support POSIX capabilities, while version 2.2 added support for POSIX capabilities to processes. When Linux documentation (including this one) says ``requires root privilege'', in nearly all cases it really means ``requires a capability'' as documented in the capability documentation. If you need to know the specific capability required, look it up in the capability documentation. In Linux, the eventual intent is to permit capabilities to be attached to files in the filesystem; as of this writing, however, this is not yet supported. There is support for transferring capabilities, but this is disabled by default. Linux version 2.2.11 added a feature that makes capabilities more directly useful, called the ``capability bounding set''. The capability bounding set is a list of capabilities that are allowed to be held by any process on the system (otherwise, only the special init process can hold it). If a capability does not appear in the bounding set, it may not be exercised by any process, no matter how privileged. This feature can be used to, for example, disable kernel module loading. A sample tool that takes advantage of this is LCAP at [http://pweb.netcom.com/~spoon/lcap/] http:// pweb.netcom.com/~spoon/lcap/. More information about POSIX capabilities is available at [ftp:// linux.kernel.org/pub/linux/libs/security/linux-privs] ftp://linux.kernel.org/ pub/linux/libs/security/linux-privs. ----------------------------------------------------------------------------- 3.1.3. Process Creation and Manipulation Processes may be created using fork(2), the non-recommended vfork(2), or the Linux-unique clone(2); all of these system calls duplicate the existing process, creating two processes out of it. A process can execute a different program by calling execve(2), or various front-ends to it (for example, see exec(3), system(3), and popen(3)). When a program is executed, and its file has its setuid or setgid bit set, the process' EUID or EGID (respectively) is usually set to the file's value. This functionality was the source of an old Unix security weakness when used to support setuid or setgid scripts, due to a race condition. Between the time the kernel opens the file to see which interpreter to run, and when the (now-set-id) interpreter turns around and reopens the file to interpret it, an attacker might change the file (directly or via symbolic links). Different Unix-like systems handle the security issue for setuid scripts in different ways. Some systems, such as Linux, completely ignore the setuid and setgid bits when executing scripts, which is clearly a safe approach. Most modern releases of SysVr4 and BSD 4.4 use a different approach to avoid the kernel race condition. On these systems, when the kernel passes the name of the set-id script to open to the interpreter, rather than using a pathname (which would permit the race condition) it instead passes the filename /dev/ fd/3. This is a special file already opened on the script, so that there can be no race condition for attackers to exploit. Even on these systems I recommend against using the setuid/setgid shell scripts language for secure programs, as discussed below. In some cases a process can affect the various UID and GID values; see setuid (2), seteuid(2), setreuid(2), and the Linux-unique setfsuid(2). In particular the saved user id (SUID) attribute is there to permit trusted programs to temporarily switch UIDs. Unix-like systems supporting the SUID use the following rules: If the RUID is changed, or the EUID is set to a value not equal to the RUID, the SUID is set to the new EUID. Unprivileged users can set their EUID from their SUID, the RUID to the EUID, and the EUID to the RUID. The Linux-unique FSUID process attribute is intended to permit programs like the NFS server to limit themselves to only the filesystem rights of some given UID without giving that UID permission to send signals to the process. Whenever the EUID is changed, the FSUID is changed to the new EUID value; the FSUID value can be set separately using setfsuid(2), a Linux-unique call. Note that non-root callers can only set FSUID to the current RUID, EUID, SEUID, or current FSUID values. ----------------------------------------------------------------------------- 3.2. Files On all Unix-like systems, the primary repository of information is the file tree, rooted at ``/''. The file tree is a hierarchical set of directories, each of which may contain filesystem objects (FSOs). In Linux, filesystem objects (FSOs) may be ordinary files, directories, symbolic links, named pipes (also called first-in first-outs or FIFOs), sockets (see below), character special (device) files, or block special (device) files (in Linux, this list is given in the find(1) command). Other Unix-like systems have an identical or similar list of FSO types. Filesystem objects are collected on filesystems, which can be mounted and unmounted on directories in the file tree. A filesystem type (e.g., ext2 and FAT) is a specific set of conventions for arranging data on the disk to optimize speed, reliability, and so on; many people use the term ``filesystem'' as a synonym for the filesystem type. ----------------------------------------------------------------------------- 3.2.1. Filesystem Object Attributes Different Unix-like systems support different filesystem types. Filesystems may have slightly different sets of access control attributes and access controls can be affected by options selected at mount time. On Linux, the ext2 filesystems is currently the most popular filesystem, but Linux supports a vast number of filesystems. Most Unix-like systems tend to support multiple filesystems too. Most filesystems on Unix-like systems store at least the following:   * owning UID and GID - identifies the ``owner'' of the filesystem object. Only the owner or root can change the access control attributes unless otherwise noted.   * permission bits - read, write, execute bits for each of user (owner), group, and other. For ordinary files, read, write, and execute have their typical meanings. In directories, the ``read'' permission is necessary to display a directory's contents, while the ``execute'' permission is sometimes called ``search'' permission and is necessary to actually enter the directory to use its contents. In a directory ``write'' permission on a directory permits adding, removing, and renaming files in that directory; if you only want to permit adding, set the sticky bit noted below. Note that the permission values of symbolic links are never used; it's only the values of their containing directories and the linked-to file that matter.   * ``sticky'' bit - when set on a directory, unlinks (removes) and renames of files in that directory are limited to the file owner, the directory owner, or root privileges. This is a very common Unix extension and is specified in the Open Group's Single Unix Specification version 2. Old versions of Unix called this the ``save program text'' bit and used this to indicate executable files that should stay in memory. Systems that did this ensured that only root could set this bit (otherwise users could have crashed systems by forcing ``everything'' into memory). In Linux, this bit has no effect on ordinary files and ordinary users can modify this bit on the files they own: Linux's virtual memory management makes this old use irrelevant.   * setuid, setgid - when set on an executable file, executing the file will set the process' effective UID or effective GID to the value of the file's owning UID or GID (respectively). All Unix-like systems support this. In Linux and System V systems, when setgid is set on a file that does not have any execute privileges, this indicates a file that is subject to mandatory locking during access (if the filesystem is mounted to support mandatory locking); this overload of meaning surprises many and is not universal across Unix-like systems. In fact, the Open Group's Single Unix Specification version 2 for chmod(3) permits systems to ignore requests to turn on setgid for files that aren't executable if such a setting has no meaning. In Linux and Solaris, when setgid is set on a directory, files created in the directory will have their GID automatically reset to that of the directory's GID. The purpose of this approach is to support ``project directories'': users can save files into such specially-set directories and the group owner automatically changes. However, setting the setgid bit on directories is not specified by standards such as the Single Unix Specification [Open Group 1997].   * timestamps - access and modification times are stored for each filesystem object. However, the owner is allowed to set these values arbitrarily (see touch(1)), so be careful about trusting this information. All Unix-like systems support this. The following are attributes are Linux-unique extensions on the ext2 filesystem, though many other filesystems have similar functionality:   * immutable bit - no changes to the filesystem object are allowed; only root can set or clear this bit. This is only supported by ext2 and is not portable across all Unix systems (or even all Linux filesystems).   * append-only bit - only appending to the filesystem object are allowed; only root can set or clear this bit. This is only supported by ext2 and is not portable across all Unix systems (or even all Linux filesystems). Other common extensions include some sort of bit indicating ``cannot delete this file''. Many of these values can be influenced at mount time, so that, for example, certain bits can be treated as though they had a certain value (regardless of their values on the media). See mount(1) for more information about this. These bits are useful, but be aware that some of these are intended to simplify ease-of-use and aren't really sufficient to prevent certain actions. For example, on Linux, mounting with ``noexec'' will disable execution of programs on that file system; as noted in the manual, it's intended for mounting filesystems containing binaries for incompatible systems. On Linux, this option won't completely prevent someone from running the files; they can copy the files somewhere else to run them, or even use the command ``/lib/ ld-linux.so.2'' to run the file directly. Some filesystems don't support some of these access control values; again, see mount(1) for how these filesystems are handled. In particular, many Unix-like systems support MS-DOS disks, which by default support very few of these attributes (and there's not standard way to define these attributes). In that case, Unix-like systems emulate the standard attributes (possibly implementing them through special on-disk files), and these attributes are generally influenced by the mount(1) command. It's important to note that, for adding and removing files, only the permission bits and owner of the file's directory really matter unless the Unix-like system supports more complex schemes (such as POSIX ACLs). Unless the system has other extensions, and stock Linux 2.2 doesn't, a file that has no permissions in its permission bits can still be removed if its containing directory permits it. Also, if an ancestor directory permits its children to be changed by some user or group, then any of that directory's descendents can be replaced by that user or group. The draft IEEE POSIX standard on security defines a technique for true ACLs that support a list of users and groups with their permissions. Unfortunately, this is not widely supported nor supported exactly the same way across Unix-like systems. Stock Linux 2.2, for example, has neither ACLs nor POSIX capability values in the filesystem. It's worth noting that in Linux, the Linux ext2 filesystem by default reserves a small amount of space for the root user. This is a partial defense against denial-of-service attacks; even if a user fills a disk that is shared with the root user, the root user has a little space left over (e.g., for critical functions). The default is 5% of the filesystem space; see mke2fs (8), in particular its ``-m'' option. ----------------------------------------------------------------------------- 3.2.2. Creation Time Initial Values At creation time, the following rules apply. On most Unix systems, when a new filesystem object is created via creat(2) or open(2), the FSO UID is set to the process' EUID and the FSO's GID is set to the process' EGID. Linux works slightly differently due to its FSUID extensions; the FSO's UID is set to the process' FSUID, and the FSO GID is set to the process' FSGUID; if the containing directory's setgid bit is set or the filesystem's ``GRPID'' flag is set, the FSO GID is actually set to the GID of the containing directory. Many systems, including Sun Solaris and Linux, also support the setgid directory extensions. As noted earlier, this special case supports ``project'' directories: to make a ``project'' directory, create a special group for the project, create a directory for the project owned by that group, then make the directory setgid: files placed there are automatically owned by the project. Similarly, if a new subdirectory is created inside a directory with the setgid bit set (and the filesystem GRPID isn't set), the new subdirectory will also have its setgid bit set (so that project subdirectories will ``do the right thing''.); in all other cases the setgid is clear for a new file. This is the rationale for the ``user-private group'' scheme (used by Red Hat Linux and some others). In this scheme, every user is a member of a ``private'' group with just themselves as members, so their defaults can permit the group to read and write any file (since they're the only member of the group). Thus, when the file's group membership is transferred this way, read and write privileges are transferred too. FSO basic access control values (read, write, execute) are computed from (requested values & ~ umask of process). New files always start with a clear sticky bit and clear setuid bit. ----------------------------------------------------------------------------- 3.2.3. Changing Access Control Attributes You can set most of these values with chmod(2), fchmod(2), or chmod(1) but see also chown(1), and chgrp(1). In Linux, some of the Linux-unique attributes are manipulated using chattr(1). Note that in Linux, only root can change the owner of a given file. Some Unix-like systems allow ordinary users to transfer ownership of their files to another, but this causes complications and is forbidden by Linux. For example, if you're trying to limit disk usage, allowing such operations would allow users to claim that large files actually belonged to some other ``victim''. ----------------------------------------------------------------------------- 3.2.4. Using Access Control Attributes Under Linux and most Unix-like systems, reading and writing attribute values are only checked when the file is opened; they are not re-checked on every read or write. Still, a large number of calls do check these attributes, since the filesystem is so central to Unix-like systems. Calls that check these attributes include open(2), creat(2), link(2), unlink(2), rename(2), mknod(2), symlink(2), and socket(2). ----------------------------------------------------------------------------- 3.2.5. Filesystem Hierarchy Over the years conventions have been built on ``what files to place where''. Where possible, please follow conventional use when placing information in the hierarchy. For example, place global configuration information in /etc. The Filesystem Hierarchy Standard (FHS) tries to define these conventions in a logical manner, and is widely used by Linux systems. The FHS is an update to the previous Linux Filesystem Structure standard (FSSTND), incorporating lessons learned and approaches from Linux, BSD, and System V systems. See [http://www.pathname.com/fhs] http://www.pathname.com/fhs for more information about the FHS. A summary of these conventions is in hier(5) for Linux and hier(7) for Solaris. Sometimes different conventions disagree; where possible, make these situations configurable at compile or installation time. I should note that the FHS has been adopted by the [http://www.linuxbase.org] Linux Standard Base which is developing and promoting a set of standards to increase compatibility among Linux distributions and to enable software applications to run on any compliant Linux system. ----------------------------------------------------------------------------- 3.3. System V IPC Many Unix-like systems, including Linux and System V systems, support System V interprocess communication (IPC) objects. Indeed System V IPC is required by the Open Group's Single UNIX Specification, Version 2 [Open Group 1997]. System V IPC objects can be one of three kinds: System V message queues, semaphore sets, and shared memory segments. Each such object has the following attributes:   * read and write permissions for each of creator, creator group, and others.   * creator UID and GID - UID and GID of the creator of the object.   * owning UID and GID - UID and GID of the owner of the object (initially equal to the creator UID). When accessing such objects, the rules are as follows:   * if the process has root privileges, the access is granted.   * if the process' EUID is the owner or creator UID of the object, then the appropriate creator permission bit is checked to see if access is granted.   * if the process' EGID is the owner or creator GID of the object, or one of the process' groups is the owning or creating GID of the object, then the appropriate creator group permission bit is checked for access.   * otherwise, the appropriate ``other'' permission bit is checked for access. Note that root, or a process with the EUID of either the owner or creator, can set the owning UID and owning GID and/or remove the object. More information is available in ipc(5). ----------------------------------------------------------------------------- 3.4. Sockets and Network Connections Sockets are used for communication, particularly over a network. Sockets were originally developed by the BSD branch of Unix systems, but they are generally portable to other Unix-like systems: Linux and System V variants support sockets as well, and socket support is required by the Open Group's Single Unix Specification [Open Group 1997]. System V systems traditionally used a different (incompatible) network communication interface, but it's worth noting that systems like Solaris include support for sockets. Socket(2) creates an endpoint for communication and returns a descriptor, in a manner similar to open(2) for files. The parameters for socket specify the protocol family and type, such as the Internet domain (TCP/IP version 4), Novell's IPX, or the ``Unix domain''. A server then typically calls bind(2), listen (2), and accept(2) or select(2). A client typically calls bind(2) (though that may be omitted) and connect(2). See these routine's respective man pages for more information. It can be difficult to understand how to use sockets from their man pages; you might want to consult other papers such as Hall "Beej" [1999] to learn how these calls are used together. The ``Unix domain sockets'' don't actually represent a network protocol; they can only connect to sockets on the same machine. (at the time of this writing for the standard Linux kernel). When used as a stream, they are fairly similar to named pipes, but with significant advantages. In particular, Unix domain socket is connection-oriented; each new connection to the socket results in a new communication channel, a very different situation than with named pipes. Because of this property, Unix domain sockets are often used instead of named pipes to implement IPC for many important services. Just like you can have unnamed pipes, you can have unnamed Unix domain sockets using socketpair(2); unnamed Unix domain sockets are useful for IPC in a way similar to unnamed pipes. There are several interesting security implications of Unix domain sockets. First, although Unix domain sockets can appear in the filesystem and can have stat(2) applied to them, you can't use open(2) to open them (you have to use the socket(2) and friends interface). Second, Unix domain sockets can be used to pass file descriptors between processes (not just the file's contents). This odd capability, not available in any other IPC mechanism, has been used to hack all sorts of schemes (the descriptors can basically be used as a limited version of the ``capability'' in the computer science sense of the term). File descriptors are sent using sendmsg(2), where the msg (message)'s field msg_control points to an array of control message headers (field msg_controllen must specify the number of bytes contained in the array). Each control message is a struct cmsghdr followed by data, and for this purpose you want the cmsg_type set to SCM_RIGHTS. A file descriptor is retrieved through recvmsg(2) and then tracked down in the analogous way. Frankly, this feature is quite baroque, but it's worth knowing about. Linux 2.2 supports an addition feature in Unix domain sockets: you can acquire the peer's ``credentials'' (the pid, uid, and gid). Here's some sample code: /* fd= file descriptor of Unix domain socket connected to the client you wish to identify */ struct ucred cr; int cl=sizeof(cr); if (getsockopt(fd, SOL_SOCKET, SO_PEERCRED, &cr, &cl)==0) { printf("Peer's pid=%d, uid=%d, gid=%d\n", cr.pid, cr.uid, cr.gid); Standard Unix convention is that binding to TCP and UDP local port numbers less than 1024 requires root privilege, while any process can bind to an unbound port number of 1024 or greater. Linux follows this convention, more specifically, Linux requires a process to have the capability CAP_NET_BIND_SERVICE to bind to a port number less than 1024; this capability is normally only held by processes with an euid of 0. The adventurous can check this in Linux by examining its Linux's source; in Linux 2.2.12, it's file /usr/src/linux/net/ipv4/af_inet.c, function inet_bind(). ----------------------------------------------------------------------------- 3.5. Signals Signals are a simple form of ``interruption'' in the Unix-like OS world, and are an ancient part of Unix. A process can set a ``signal'' on another process (say using kill(1) or kill(2)), and that other process would receive and handle the signal asynchronously. For a process to have permission to send a signal to some other process, the sending process must either have root privileges, or the real or effective user ID of the sending process must equal the real or saved set-user-ID of the receiving process. Although signals are an ancient part of Unix, they've had different semantics in different implementations. Basically, they involve questions such as ``what happens when a signal occurs while handling another signal''? The older Linux libc 5 used a different set of semantics for some signal operations than the newer GNU libc libraries. Calling C library functions is not necessarily safe within a signal handler; you need to examine the documentation for each call you make. For more information, see the glibc FAQ (on some systems a local copy is available at /usr/doc/glibc-*/FAQ). For new programs, just use the POSIX signal system (which in turn was based on BSD work); this set is widely supported and doesn't have the problems that some of the older signal systems did. The POSIX signal system is based on using the sigset_t datatype, which can be manipulated through a set of operations: sigemptyset(), sigfillset(), sigaddset(), sigdelset(), and sigismember(). You can read about these in sigsetops(3). Then use sigaction (2), sigaction(2), sigprocmask(2), sigpending(2), and sigsuspend(2) to set up an manipulate signal handling (see their man pages for more information). In general, make any signal handlers very short and simple, and look carefully for race conditions. Signals, since they are by nature asynchronous, can easily cause race conditions. A common convention exists for servers: if you receive SIGHUP, you should close any log files, reopen and reread configuration files, and then re-open the log files. This supports reconfiguration without halting the server and log rotation without data loss. If you are writing a server where this convention makes sense, please support it. ----------------------------------------------------------------------------- 3.6. Quotas and Limits Many Unix-like systems have mechanisms to support filesystem quotas and process resource limits. This certainly includes Linux. These mechanisms are particularly useful for preventing denial of service attacks; by limiting the resources available to each user, you can make it hard for a single user to use up all the system resources. Be careful with terminology here, because both filesystem quotas and process resource limits have ``hard'' and ``soft'' limits but the terms mean slightly different things. You can define storage (filesystem) quota limits on each mountpoint for the number of blocks of storage and/or the number of unique files (inodes) that can be used, and you can set such limits for a given user or a given group. A ``hard'' quota limit is a never-to-exceed limit, while a ``soft'' quota can be temporarily exceeded. See quota(1), quotactl(2), and quotaon(8). The rlimit mechanism supports a large number of process quotas, such as file size, number of child processes, number of open files, and so on. There is a ``soft'' limit (also called the current limit) and a ``hard limit'' (also called the upper limit). The soft limit cannot be exceeded at any time, but through calls it can be raised up to the value of the hard limit. See getrlimit(2), setrlimit(2), and getrusage(2), sysconf(3), and ulimit(1). Note that there are several ways to set these limits, including the PAM module pam_limits. ----------------------------------------------------------------------------- 3.7. Dynamically Linked Libraries Practically all programs depend on libraries to execute. In most modern Unix-like systems, including Linux, programs are by default compiled to use dynamically linked libraries (DLLs). That way, you can update a library and all the programs using that library will use the new (hopefully improved) version if they can. Dynamically linked libraries are typically placed in one a few special directories. The usual directories include /lib, /usr/lib, /lib/security for PAM modules, /usr/X11R6/lib for X-windows, and /usr/local/lib. You should use these standard conventions in your programs, in particular, except during debugging you shouldn't use value computed from the current directory as a source for dynamically linked libraries (an attacker may be able to add their own choice ``library'' values). There are special conventions for naming libraries and having symbolic links for them, with the result that you can update libraries and still support programs that want to use old, non-backward-compatible versions of those libraries. There are also ways to override specific libraries or even just specific functions in a library when executing a particular program. This is a real advantage of Unix-like systems over Windows-like systems; I believe Unix-like systems have a much better system for handling library updates, one reason that Unix and Linux systems are reputed to be more stable than Windows-based systems. On GNU glibc-based systems, including all Linux systems, the list of directories automatically searched during program start-up is stored in the file /etc/ld.so.conf. Many Red Hat-derived distributions don't normally include /usr/local/lib in the file /etc/ld.so.conf. I consider this a bug, and adding /usr/local/lib to /etc/ld.so.conf is a common ``fix'' required to run many programs on Red Hat-derived systems. If you want to just override a few functions in a library, but keep the rest of the library, you can enter the names of overriding libraries (.o files) in /etc/ld.so.preload; these ``preloading'' libraries will take precedence over the standard set. This preloading file is typically used for emergency patches; a distribution usually won't include such a file when delivered. Searching all of these directories at program start-up would be too time-consuming, so a caching arrangement is actually used. The program ldconfig(8) by default reads in the file /etc/ld.so.conf, sets up the appropriate symbolic links in the dynamic link directories (so they'll follow the standard conventions), and then writes a cache to /etc/ld.so.cache that's then used by other programs. So, ldconfig has to be run whenever a DLL is added, when a DLL is removed, or when the set of DLL directories changes; running ldconfig is often one of the steps performed by package managers when installing a library. On start-up, then, a program uses the dynamic loader to read the file /etc/ld.so.cache and then load the libraries it needs. Various environment variables can control this process, and in fact there are environment variables that permit you to override this process (so, for example, you can temporarily substitute a different library for this particular execution). In Linux, the environment variable LD_LIBRARY_PATH is a colon-separated set of directories where libraries are searched for first, before the standard set of directories; this is useful when debugging a new library or using a nonstandard library for special purposes, but be sure you trust those who can control those directories. The variable LD_PRELOAD lists object files with functions that override the standard set, just as /etc/ ld.so.preload does. The variable LD_DEBUG, displays debugging information; if set to ``all'', voluminous information about the dynamic linking process is displayed while it's occurring. Permitting user control over dynamically linked libraries would be disastrous for setuid/setgid programs if special measures weren't taken. Therefore, in the GNU glibc implementation, if the program is setuid or setgid these variables (and other similar variables) are ignored or greatly limited in what they can do. The GNU glibc library determines if a program is setuid or setgid by checking the program's credentials; if the uid and euid differ, or the gid and the egid differ, the library presumes the program is setuid/ setgid (or descended from one) and therefore greatly limits its abilities to control linking. If you load the GNU glibc libraries, you can see this; see especially the files elf/rtld.c and sysdeps/generic/dl-sysdep.c. This means that if you cause the uid and gid to equal the euid and egid, and then call a program, these variables will have full effect. Other Unix-like systems handle the situation differently but for the same reason: a setuid/setgid program should not be unduly affected by the environment variables set. For Linux systems, you can get more information from my document, the Program Library HOWTO. ----------------------------------------------------------------------------- 3.8. Audit Different Unix-like systems handle auditing differently. In Linux, the most common ``audit'' mechanism is syslogd(8), usually working in conjuction with klogd(8). You might also want to look at wtmp(5), utmp(5), lastlog(8), and acct(2). Some server programs (such as the Apache web server) also have their own audit trail mechanisms. According to the FHS, audit logs should be stored in /var/log or its subdirectories. ----------------------------------------------------------------------------- 3.9. PAM Sun Solaris and nearly all Linux systems use the Pluggable Authentication Modules (PAM) system for authentication. PAM permits run-time configuration of authentication methods (e.g., use of passwords, smart cards, etc.). See Section 10.5 for more information on using PAM. ----------------------------------------------------------------------------- 3.10. Specialized Security Extensions for Unix-like Systems A vast amount of research and development has gone into extending Unix-like systems to support security needs of various communities. For example, several Unix-like systems have been extended to support the U.S. military's desire for multilevel security. If you're developing software, you should try to design your software so that it can work within these extensions. FreeBSD has a new system call, [http://docs.freebsd.org/44doc/papers/jail/ jail.html] jail(2). The jail system call supports sub-partioning an environment into many virtual machines (in a sense, a ``super-chroot''); its most popular use has been to provide virtual machine services for Internet Service Provider environments. Inside a jail, all processes (even those owned by root) have the the scope of their requests limited to the jail. When a FreeBSD system is booted up after a fresh install, no processes will be in jail. When a process is placed in a jail, it, and any descendents of that process created will be in that jail. Once in a jail, access to the file name-space is restricted in the style of chroot(2) (with typical chroot escape routes blocked), the ability to bind network resources is limited to a specific IP address, the ability to manipulate system resources and perform privileged operations is sharply curtailed, and the ability to interact with other processes is limited to only processes inside the same jail. Note that each jail is bound to a single IP address; processes within the jail may not make use of any other IP address for outgoing or incoming connections. Some extensions available in Linux, such as POSIX capabilities and special mount-time options, have already been discussed. Here are a few of these efforts for Linux systems for creating restricted execution environments; there are many different approaches. The U.S. National Security Agency (NSA) has developed [http://www.nsa.gov/selinux] Security-Enhanced Linux (Flask), which supports defining a security policy in a specialized language and then enforces that policy. The [http://medusa.fornax.sk] Medusa DS9 extends Linux by supporting, at the kernel level, a user-space authorization server. [http: //www.lids.org] LIDS protects files and processes, allowing administrators to ``lock down'' their system. The ``Rule Set Based Access Control'' system, [http://www.rsbac.de] RSBAC is based on the Generalized Framework for Access Control (GFAC) by Abrams and LaPadula and provides a flexible system of access control based on several kernel modules. [http://subterfugue.org] Subterfugue is a framework for ``observing and playing with the reality of software''; it can intercept system calls and change their parameters and/or change their return values to implement sandboxes, tracers, and so on; it runs under Linux 2.4 with no changes (it doesn't require any kernel modifications). [http://www.cs.berkeley.edu/~daw/janus] Janus is a security tool for sandboxing untrusted applications within a restricted execution environment. Some have even used [http://user-mode-linux.sourceforge.net] User-mode Linux, which implements ``Linux on Linux'', as a sandbox implementation. There are many other extensions for security on various Unix-like systems, but these are really outside the scope of this document. ----------------------------------------------------------------------------- Chapter 4. Validate All Input   Wisdom will save you from the ways of wicked men, from men whose words are perverse...   Proverbs 2:12 (NIV) Some inputs are from untrustable users, so those inputs must be validated (filtered) before being used. You should determine what is legal and reject anything that does not match that definition. Do not do the reverse (identify what is illegal and write code to reject those cases), because you are likely to forget to handle an important case of illegal input. There is a good reason for identifying ``illegal'' values, though, and that's as a set of tests (usually just executed in your head) to be sure that your validation code is thorough. When I set up an input filter, I mentally attack the filter to see if there are illegal values that could get through. Depending on the input, here are a few examples of common ``illegal'' values that your input filters may need to prevent: the empty string, ".", "..", ".. /", anything starting with "/" or ".", anything with "/" or "&" inside it, any control characters (especially NIL and newline), and/or any characters with the ``high bit'' set (especially values decimal 254 and 255). Again, your code should not be checking for ``bad'' values; you should do this check mentally to be sure that your pattern ruthlessly limits input values to legal values. If your pattern isn't sufficiently narrow, you need to carefully re-examine the pattern to see if there are other problems. Limit the maximum character length (and minimum length if appropriate), and be sure to not lose control when such lengths are exceeded (see Chapter 5 for more about buffer overflows). For strings, identify the legal characters or legal patterns (e.g., as a regular expression) and reject anything not matching that form. There are special problems when strings contain control characters (especially linefeed or NIL) or shell metacharacters; it is often best to ``escape'' such metacharacters immediately when the input is received so that such characters are not accidentally sent. CERT goes further and recommends escaping all characters that aren't in a list of characters not needing escaping [CERT 1998, CMU 1998]. See Section 7.2 for more information on limiting call-outs. Limit all numbers to the minimum (often zero) and maximum allowed values. A full email address checker is actually quite complicated, because there are legacy formats that greatly complicate validation if you need to support all of them; see mailaddr(7) and IETF RFC 822 [RFC 822] for more information if such checking is necessary. Filenames should be checked; usually you will want to not include ``..'' (higher directory) as a legal value. In filenames it's best to prohibit any change in directory, e.g., by not including ``/'' in the set of legal characters. Often you shouldn't support ``globbing'', that is, expanding filenames using ``*'', ``?'', ``['' (matching ``]''), and possibly ``{'' (matching ``}''). For example, the command ``ls *.png'' does a glob on ``*.png'' to list all PNG files. The C fopen(3) command (for example) doesn't do globbing, but the command shells perform globbing by default, and in C you can request globbing using (for example) glob(3). If you don't need globbing, just use the calls that don't do it where possible (e.g., fopen(3)) and/or disable them (e.g., escape the globbing characters in a shell). Be especially careful if you want to permit globbing. Globbing can be useful, but complex globs can take a great deal of computing time. For example, on some ftp servers, performing a few of these requests can easily cause a denial-of-service of the entire machine: ftp> ls */../*/../*/../*/../*/../*/../*/../*/../*/../*/../*/../*/../* Trying to allow globbing, yet limit globbing patterns, is probably futile. Insted, make sure that any such programs run as a seperate process and use process limits to limit the amount of CPU and other resources they can consume. See Section 6.3.8 for more information on this approach, and see Section 3.6 for more information on how to set these limits. Unless you account for them, the legal character patterns must not include characters or character sequences that have special meaning to either the program internals or the eventual output:   * A character sequence may have special meaning to the program's internal storage format. For example, if you store data (internally or externally) in delimited strings, make sure that the delimiters are not permitted data values. A number of programs store data in comma (,) or colon (:) delimited text files; inserting the delimiters in the input can be a problem unless the program accounts for it (i.e., by preventing it or encoding it in some way). Other characters often causing these problems include single and double quotes (used for surrounding strings) and the less-than sign "<" (used in SGML, XML, and HTML to indicate a tag's beginning; this is important if you store data in these formats). Most data formats have an escape sequence to handle these cases; use it, or filter such data on input.   * A character sequence may have special meaning if sent back out to a user. A common example of this is permitting HTML tags in data input that will later be posted to other readers (e.g., in a guestbook or ``reader comment'' area). However, the problem is much more general. See Section 6.13 for a general discussion on the topic, and see Section 4.10 for a specific discussion about filtering HTML. These tests should usually be centralized in one place so that the validity tests can be easily examined for correctness later. Make sure that your validity test is actually correct; this is particularly a problem when checking input that will be used by another program (such as a filename, email address, or URL). Often these tests are have subtle errors, producing the so-called ``deputy problem'' (where the checking program makes different assumptions than the program that actually uses the data). If there's a relevant standard, look at it, but also search to see if the program has extensions that you need to know about. While parsing user input, it's a good idea to temporarily drop all privileges, or even create separate processes (with the parser having permanently dropped privileges, and the other process performing security checks against the parser requests). This is especially true if the parsing task is complex (e.g., if you use a lex-like or yacc-like tool), or if the programming language doesn't protect against buffer overflows (e.g., C and C++). See Section 6.3 for more information on minimizing privileges. When using data for security decisions (e.g., ``let this user in''), be sure to use trustworthy channels. For example, on a public Internet, don't just use the machine IP address or port number as the sole way to authenticate users, because in most environments this information can be set by the (potentially malicious) user. See Section 6.9 for more information. The following subsections discuss different kinds of inputs to a program; note that input includes process state such as environment variables, umask values, and so on. Not all inputs are under the control of an untrusted user, so you need only worry about those inputs that are. ----------------------------------------------------------------------------- 4.1. Command line Many programs take input from the command line. A setuid/setgid program's command line data is provided by an untrusted user, so a setuid/setgid program must defend itself from potentially hostile command line values. Attackers can send just about any kind of data through a command line (through calls such as the execve(3) call). Therefore, setuid/setgid programs must completely validate the command line inputs and must not trust the name of the program reported by command line argument zero (an attacker can set it to any value including NULL). ----------------------------------------------------------------------------- 4.2. Environment Variables By default, environment variables are inherited from a process' parent. However, when a program executes another program, the calling program can set the environment variables to arbitrary values. This is dangerous to setuid/ setgid programs, because their invoker can completely control the environment variables they're given. Since they are usually inherited, this also applies transitively; a secure program might call some other program and, without special measures, would pass potentially dangerous environment variables values on to the program it calls. The following subsections discuss environment variables and what to do with them. ----------------------------------------------------------------------------- 4.2.1. Some Environment Variables are Dangerous Some environment variables are dangerous because many libraries and programs are controlled by environment variables in ways that are obscure, subtle, or undocumented. For example, the IFS variable is used by the sh and bash shell to determine which characters separate command line arguments. Since the shell is invoked by several low-level calls (like system(3) and popen(3) in C, or the back-tick operator in Perl), setting IFS to unusual values can subvert apparently-safe calls. This behavior is documented in bash and sh, but it's obscure; many long-time users only know about IFS because of its use in breaking security, not because it's actually used very often for its intended purpose. What is worse is that not all environment variables are documented, and even if they are, those other programs may change and add dangerous environment variables. Thus, the only real solution (described below) is to select the ones you need and throw away the rest. ----------------------------------------------------------------------------- 4.2.2. Environment Variable Storage Format is Dangerous Normally, programs should use the standard access routines to access environment variables. For example, in C, you should get values using getenv (3), set them using the POSIX standard routine putenv(3) or the BSD extension setenv(3) and eliminate environment variables using unsetenv(3). I should note here that setenv(3) is implemented in Linux, too. However, crackers need not be so nice; crackers can directly control the environment variable data area passed to a program using execve(2). This permits some nasty attacks, which can only be understood by understanding how environment variables really work. In Linux, you can see environ(5) for a summary how about environment variables really work. In short, environment variables are internally stored as a pointer to an array of pointers to characters; this array is stored in order and terminated by a NULL pointer (so you'll know when the array ends). The pointers to characters, in turn, each point to a NIL-terminated string value of the form ``NAME=value''. This has several implications, for example, environment variable names can't include the equal sign, and neither the name nor value can have embedded NIL characters. However, a more dangerous implication of this format is that it allows multiple entries with the same variable name, but with different values (e.g., more than one value for SHELL). While typical command shells prohibit doing this, a locally-executing cracker can create such a situation using execve(2). The problem with this storage format (and the way it's set) is that a program might check one of these values (to see if it's valid) but actually use a different one. In Linux, the GNU glibc libraries try to shield programs from this; glibc 2.1's implementation of getenv will always get the first matching entry, setenv and putenv will always set the first matching entry, and unsetenv will actually unset all of the matching entries (congratulations to the GNU glibc implementors for implementing unsetenv this way!). However, some programs go directly to the environ variable and iterate across all environment variables; in this case, they might use the last matching entry instead of the first one. As a result, if checks were made against the first matching entry instead, but the actual value used is the last matching entry, a cracker can use this fact to circumvent the protection routines. ----------------------------------------------------------------------------- 4.2.3. The Solution - Extract and Erase For secure setuid/setgid programs, the short list of environment variables needed as input (if any) should be carefully extracted. Then the entire environment should be erased, followed by resetting a small set of necessary environment variables to safe values. There really isn't a better way if you make any calls to subordinate programs; there's no practical method of listing ``all the dangerous values''. Even if you reviewed the source code of every program you call directly or indirectly, someone may add new undocumented environment variables after you write your code, and one of them may be exploitable. The simple way to erase the environment in C/C++ is by setting the global variable environ to NULL. The global variable environ is defined in ; C/C++ users will want to #include this header file. You will need to manipulate this value before spawning threads, but that's rarely a problem, since you want to do these manipulations very early in the program's execution (usually before threads are spawned). The global variable environ's definition is defined in various standards; it's not clear that the official standards condone directly changing its value, but I'm unaware of any Unix-like system that has trouble with doing this. I normally just modify the ``environ'' directly; manipulating such low-level components is possibly non-portable, but it assures you that you get a clean (and safe) environment. In the rare case where you need later access to the entire set of variables, you could save the ``environ'' variable's value somewhere, but this is rarely necessary; nearly all programs need only a few values, and the rest can be dropped. Another way to clear the environment is to use the undocumented clearenv() function. The function clearenv() has an odd history; it was supposed to be defined in POSIX.1, but somehow never made it into that standard. However, clearenv() is defined in POSIX.9 (the Fortran 77 bindings to POSIX), so there is a quasi-official status for it. In Linux, clearenv() is defined in < stdlib.h>, but before using #include to include it you must make sure that __USE_MISC is #defined. A somewhat more ``official'' approach is to cause __USE_MISC to be defined is to #define either _SVID_SOURCE or _BSD_SOURCE, then #include - these are the official feature test macros. One environment value you'll almost certainly re-add is PATH, the list of directories to search for programs; PATH should not include the current directory and usually be something simple like ``/bin:/usr/bin''. Typically you'll also set IFS (to its default of `` \t\n'', where space is the first character) and TZ (timezone). Linux won't die if you don't supply either IFS or TZ, but some System V based systems have problems if you don't supply a TZ value, and it's rumored that some shells need the IFS value set. In Linux, see environ(5) for a list of common environment variables that you might want to set. If you really need user-supplied values, check the values first (to ensure that the values match a pattern for legal values and that they are within some reasonable maximum length). Ideally there would be some standard trusted file in /etc with the information for ``standard safe environment variable values'', but at this time there's no standard file defined for this purpose. For something similar, you might want to examine the PAM module pam_env on those systems which have that module. If you're using a shell as your programming language, you can use the ``/usr/ bin/env'' program with the ``-'' option (which erases all environment variables of the program being run). Basically, you call /usr/bin/env, give it the ``-'' option, follow that with the set of variables and their values you wish to set (as name=value), and then follow that with the name of the program to run and its arguments. You usually want to call the program using the full pathname (/usr/bin/env) and not just as ``env'', in case a user has created a dangerous PATH value. Note that GNU's env also accepts the options "-i" and "--ignore-environment" as synonyms (they also erase the environment of the program being started), but these aren't portable to other versions of env. If you're programming a setuid/setgid program in a language that doesn't allow you to reset the environment directly, one approach is to create a ``wrapper'' program. The wrapper sets the environment program to safe values, and then calls the other program. Beware: make sure the wrapper will actually invoke the intended program; if it's an interpreted program, make sure there's no race condition possible that would allow the interpreter to load a different program than the one that was granted the special setuid/setgid privileges. ----------------------------------------------------------------------------- 4.3. File Descriptors A program is passed a set of ``open file descriptors'', that is, pre-opened files. A setuid/setgid program must deal with the fact that the user gets to select what files are open and to what (within their permission limits). A setuid/setgid program must not assume that opening a new file will always open into a fixed file descriptor id. It must also not assume that standard input (stdin), standard output (stdout), and standard error (stderr) refer to a terminal or are even open. The rationale behind this is easy; since an attacker can open or close a file descriptor before starting the program, the attacker could create an unexpected situation. If the attacker closes the standard output, when the program opens the next file it will be opened as though it were standard output, and then it will send all standard output to that file as well. Some C libraries will automatically open stdin, stdout, and stderr if they aren't already open (to /dev/null), but this isn't true on all Unix-like systems. ----------------------------------------------------------------------------- 4.4. File Contents If a program takes directions from a file, it must not trust that file specially unless only a trusted user can control its contents. Usually this means that an untrusted user must not be able to modify the file, its directory, or any of its ancestor directories. Otherwise, the file must be treated as suspect. If the directions in the file are supposed to be from an untrusted user, then make sure that the inputs from the file are protected as describe throughout this book. In particular, check that values match the set of legal values, and that buffers are not overflowed. ----------------------------------------------------------------------------- 4.5. Web-Based Application Inputs (Especially CGI Scripts) Web-based applications (such as CGI scripts) run on some trusted server and must get their input data somehow through the web. Since the input data generally come from untrusted users, this input data must be validated. Indeed, this information may have actually come from an untrusted third party; see Section 6.13 for more information. For example, CGI scripts are passed this information through a standard set of environment variables and through standard input. The rest of this text will specifically discuss CGI, because it's the most common technique for implementing dynamic web content, but the general issues are the same for most other dynamic web content techniques. One additional complication is that many CGI inputs are provided in so-called ``URL-encoded'' format, that is, some values are written in the format %HH where HH is the hexadecimal code for that byte. You or your CGI library must handle these inputs correctly by URL-decoding the input and then checking if the resulting byte value is acceptable. You must correctly handle all values, including problematic values such as %00 (NIL) and %0A (newline). Don't decode inputs more than once, or input such as ``%2500'' will be mishandled (the %25 would be translated to ``%'', and the resulting ``%00'' would be erroneously translated to the NIL character). CGI scripts are commonly attacked by including special characters in their inputs; see the comments above. Another form of data available to web-based applications are ``cookies.'' Again, users can provide arbitrary cookie values, so they cannot be trusted unless special precautions are taken. Also, cookies can be used to track users, creating a privacy problem for many users. As a result, many users disable cookies, so if possible your web application should be designed so that it does not require the use of cookies. Some HTML forms include client-side input checking to prevent some illegal values; these are typically implemented using Javascript/ECMAscript or Java. This checking can be helpful for the user, since it can happen ``immediately'' without requiring any network access. However, this kind of input checking is useless for security, because attackers can send such ``illegal'' values directly to the web server without going through the checks. It's not even hard to subvert this; you don't have to write a program to send arbitrary data to a web application. In general, servers must perform all their own input checking (of form data, cookies, and so on) because they cannot trust clients to do this securely. In short, clients are generally not ``trustworthy channels''. See Section 6.9 for more information on trustworthy channels. ----------------------------------------------------------------------------- 4.6. Other Inputs Programs must ensure that all inputs are controlled; this is particularly difficult for setuid/setgid programs because they have so many such inputs. Other inputs programs must consider include the current directory, signals, memory maps (mmaps), System V IPC, and the umask (which determines the default permissions of newly-created files). Consider explicitly changing directories (using chdir(2)) to an appropriately fully named directory at program startup. ----------------------------------------------------------------------------- 4.7. Human Language (Locale) Selection As more people have computers and the Internet available to them, there has been increasing pressure for programs to support multiple human languages and cultures. This combination of language and other cultural factors is usually called a ``locale''. The process of modifying a program so it can support multiple locales is called ``internationalization'' (i18n), and the process of providing the information for a particular locale to a program is called ``localization'' (l10n). Overall, internationalization is a good thing, but this process provides another opportunity for a security exploit. Since a potentially untrusted user provides information on the desired locale, locale selection becomes another input that, if not properly protected, can be exploited. ----------------------------------------------------------------------------- 4.7.1. How Locales are Selected In locally-run programs (including setuid/setgid programs), locale information is provided by an environment variable. Thus, like all other environment variables, these values must be extracted and checked against valid patterns before use. For web applications, this information can be obtained from the web browser (via the Accept-Language request header). However, since not all web browsers properly pass this information (and not all users configure their browsers properly), this is used less often than you might think. Often, the language requested in a web browser is simply passed in as a form value. Again, these values must be checked for validity before use, as with any other form value. In either case, locale information is really just a special case of input discussed in the previous sections. However, because this input is so rarely considered, I'm discussing it separately. In particular, when combined with format strings (discussed later), user-controlled strings can permit attackers to force other programs to run arbitrary instructions, corrupt data, and do other unfortunate actions. ----------------------------------------------------------------------------- 4.7.2. Locale Support Mechanisms There are two major library interfaces for supporting locale-selected messages on Unix-like systems, one called ``catgets'' and the other called ``gettext''. In the catgets approach, every string is assigned a unique number, which is used as an index into a table of messages. In contrast, in the gettext approach, a string (usually in English) is used to look up a table that translates the original string. catgets(3) is an accepted standard (via the X/Open Portability Guide, Volume 3 and Single Unix Specification), so it's possible your program uses it. The ``gettext'' interface is not an official standard, (though it was originally a UniForum proposal), but I believe it's the more widely used interface (it's used by Sun and essentially all GNU programs). In theory, catgets should be slightly faster, but this is at best marginal on today's machines, and the bookkeeping effort to keep unique identifiers valid in catgets() makes the gettext() interface much easier to use. I'd suggest using gettext(), just because it's easier to use. However, don't take my word for it; see GNU's documentation on gettext (info:gettext#catgets) for a longer and more descriptive comparison. The catgets(3) call (and its associated catopen(3) call) in particular is vulnerable to security problems, because the environment variable NLSPATH can be used to control the filenames used to acquire internationalized messages. The GNU C library ignores NLSPATH for setuid/setgid programs, which helps, but that doesn't protect programs running on other implementations, nor other programs (like CGI scripts) which don't ``appear'' to require such protection. The widely-used ``gettext'' interface is at least not vulnerable to a malicious NLSPATH setting to my knowledge. However, it appears likely to me that malicious settings of LC_ALL or LC_MESSAGES could cause problems. Also, if you use gettext's bindtextdomain() routine in its file cat-compat.c, that does depend on NLSPATH. ----------------------------------------------------------------------------- 4.7.3. Legal Values For the moment, if you must permit untrusted users to set information on their desired locales, make sure the provided internationalization information meets a narrow filter that only permits legitimate locale names. For user programs (especially setuid/setgid programs), these values will come in via NLSPATH, LANGUAGE, LANG, the old LINGUAS, LC_ALL, and the other LC_* values (especially LC_MESSAGES, but also including LC_COLLATE, LC_CTYPE, LC_MONETARY, LC_NUMERIC, and LC_TIME). For web applications, this user-requested set of language information would be done via the Accept-Language request header or a form value (the application should indicate the actual language setting of the data being returned via the Content-Language heading). You can check this value as part of your environment variable filtering if your users can set your environment variables (i.e., setuid/setgid programs) or as part of your input filtering (e.g., for CGI scripts). The GNU C library "glibc" doesn't accept some values of LANG for setuid/setgid programs (in particular anything with "/"), but errors have been found in that filtering (e.g., Red Hat released an update to fix this error in glibc on September 1, 2000). This kind of filtering isn't required by any standard, so you're safer doing this filtering yourself. I have not found any guidance on filtering language settings, so here are my suggestions based on my own research into the issue. First, a few words about the legal values of these settings. Language settings are generally set using the standard tags defined in IETF RFC 1766 (which uses two-letter country codes as its basic tag, followed by an optional subtag separated by a dash; I've found that environment variable settings use the underscore instead). However, some find this insufficiently flexible, so three-letter country codes may soon be used as well. Also, there are two major not-quite compatible extended formats, the X/Open Format and the CEN Format (European Community Standard); you'd like to permit both. Typical values include ``C'' (the C locale), ``EN'' (English''), and ``FR_fr'' (French using the territory of France's conventions). Also, so many people use nonstandard names that programs have had to develop ``alias'' systems to cope with them (for GNU gettext, see /usr/share/locale/ locale.aliases, and for X11, see /usr/lib/X11/locale/locale.aliases); they should usually be permitted as well. Libraries like gettext() have to accept all these variants and find an appropriate value, where possible. One source of further information is FSF [1999]. However, a filter should not permit characters that aren't needed, in particular ``/'' (which might permit escaping out of the trusted directories) and ``..'' (which might permit going up one directory). Other dangerous characters in NLSPATH include ``%'' (which indicates substitution) and ``:'' (which is the directory separator); the documentation I have for other machines suggests that some implementations may use them for other values, so it's safest to prohibit them. ----------------------------------------------------------------------------- 4.7.4. Bottom Line In short, I suggest simply erasing or re-setting the NLSPATH, unless you have a trusted user supplying the value. For the Accept-Language heading in HTTP (if you use it), form values specifying the locale, and the environment variables LANGUAGE, LANG, the old LINGUAS, LC_ALL, and the other LC_* values listed above, filter the locales from untrusted users to permit null (empty) values or to only permit values that match in total this regular expression: [A-Za-z][A-Za-z0-9_,+@\-\.]* I haven't found any legitimate locale which doesn't match this pattern, but this pattern does appear to protect against locale attacks. Of course, there's no guarantee that there are messages available in the requested locale, but in such a case these routines will fall back to the default messages (usually in English), which at least is not a security problem. Of course, languages cannot be supported without a standard way to represent their written symbols, which brings us to the issue of character encoding. ----------------------------------------------------------------------------- 4.8. Character Encoding 4.8.1. Introduction to Character Encoding For many years Americans have exchanged text using the ASCII character set; since essentially all U.S. systems support ASCII, this permits easy exchange of English text. Unfortunately, ASCII is completely inadequate in handling the characters of nearly all other languages. For many years different countries have adopted different techniques for exchanging text in different languages, making it difficult to exchange data in an increasingly interconnected world. More recently, ISO has developed ISO 10646, the ``Universal Mulitple-Octet Coded Character Set (UCS). UCS is a coded character set which defines a single 31-bit value for each of all of the world's characters. The first 65536 characters of the UCS (which thus fit into 16 bits) are termed the ``Basic Multilingual Plane'' (BMP), and the BMP is intended to cover nearly all of today's spoken languages. The Unicode forum develops the Unicode standard, which concentrates on the UCS and adds some additional conventions to aid interoperability. Historically, Unicode and ISO 10646 were developed by competing groups, but thankfully they realized that they needed to work together and they now coordinate with each other. If you're writing new software that handles internationalized characters, you should be using ISO 10646/Unicode as your basis for handling international characters. However, you may need to process older documents in various older (language-specific) character sets, in which case, you need to ensure that an untrusted user cannot control the setting of another document's character set (since this would significantly affect the document's interpretation). ----------------------------------------------------------------------------- 4.8.2. Introduction to UTF-8 Most software is not designed to handle 16 bit or 32 bit characters, yet to create a universal character set more than 8 bits was required. Therefore, a special format called ``UTF-8'' was developed to encode these potentially international characters in a format more easily handled by existing programs and libraries. UTF-8 is defined, among other places, in IETF RFC 2279, so it's a well-defined standard that can be freely read and used. UTF-8 is a variable-width encoding; characters numbered 0 to 0x7f (127) encode to themselves as a single byte, while characters with larger values are encoded into 2 to 6 bytes of information (depending on their value). The encoding has been specially designed to have the following nice properties (this information is from the RFC and Linux utf-8 man page):   * The classical US ASCII characters (0 to 0x7f) encode as themselves, so files and strings which contain only 7-bit ASCII characters have the same encoding under both ASCII and UTF-8. This is fabulous for backwards compatibility with the many existing U.S. programs and data files.   * All UCS characters beyond 0x7f are encoded as a multibyte sequence consisting only of bytes in the range 0x80 to 0xfd. This means that no ASCII byte can appear as part of another character. Many other encodings permit characters such as an embedded NIL, causing programs to fail.   * It's easy to convert between UTF-8 and a 2-byte or 4-byte fixed-width representations of characters (these are called UCS-2 and UCS-4 respectively).   * The lexicographic sorting order of UCS-4 strings is preserved, and the Boyer-Moore fast search algorithm can be used directly with UTF-8 data.   * All possible 2^31 UCS codes can be encoded using UTF-8.   * The first byte of a multibyte sequence which represents a single non-ASCII UCS character is always in the range 0xc0 to 0xfd and indicates how long this multibyte sequence is. All further bytes in a multibyte sequence are in the range 0x80 to 0xbf. This allows easy resynchronization; if a byte is missing, it's easy to skip forward to the ``next'' character, and it's always easy to skip forward and back to the ``next'' or ``preceding'' character. In short, the UTF-8 transformation format is becoming a dominant method for exchanging international text information because it can support all of the world's languages, yet it is backward compatible with U.S. ASCII files as well as having other nice properties. For many purposes I recommend its use, particularly when storing data in a ``text'' file. ----------------------------------------------------------------------------- 4.8.3. UTF-8 Security Issues The reason to mention UTF-8 is that some byte sequences are not legal UTF-8, and this might be an exploitable security hole. UTF-8 encoders are supposed to use the ``shortest possible'' encoding, but naive decoders may accept encodings that are longer than necessary. If an attacker intentionally creates an unusually long format, input checkers might not notice the problem. The RFC describes the problem this way: Implementors of UTF-8 need to consider the security aspects of how they handle illegal UTF-8 sequences. It is conceivable that in some circumstances an attacker would be able to exploit an incautious UTF-8 parser by sending it an octet sequence that is not permitted by the UTF-8 syntax. A particularly subtle form of this attack could be carried out against a parser which performs security-critical validity checks against the UTF-8 encoded form of its input, but interprets certain illegal octet sequences as characters. For example, a parser might prohibit the NUL character when encoded as the single-octet sequence 00, but allow the illegal two-octet sequence C0 80 (illegal because it's longer than necessary) and interpret it as a NUL character (00). Another example might be a parser which prohibits the octet sequence 2F 2E 2E 2F ("/../"), yet permits the illegal octet sequence 2F C0 AE 2E 2F. A longer discussion about this is available at Markus Kuhn's UTF-8 and Unicode FAQ for Unix/Linux at [http://www.cl.cam.ac.uk/~mgk25/unicode.html] http://www.cl.cam.ac.uk/~mgk25/unicode.html. ----------------------------------------------------------------------------- 4.8.4. UTF-8 Legal Values Thus, when accepting UTF-8 input, you need to check if it's valid UTF-8. Here is a list of all legal UTF-8 sequences; any character sequence not matching this table is not a legal UTF-8 sequence. In the following table, the first three rows list the legal UTF-8 sequences in binary, hexadecimal, and octal. The last row lists the UCS code region that the sequence encodes to (in hexadecimal). In the binary column, the ``x'' indicates that either a 0 or 1 is legal in the sequence. In the other columns, a ``-'' indicates a range of legal values (inclusive). Of course, just because a sequence is a legal UTF-8 sequence doesn't mean that you should accept it (see the other issues discussed in this book). Table 4-1. Legal UTF-8 Sequences +---------------+--------------+--------------+--------------+-----------------+ |Binary |Hexadecimal |Octal |Decimal |UCS Code (Hex) | +---------------+--------------+--------------+--------------+-----------------+ |0xxxxxxx |00-7F |0-177 |0-127 |0000000-0000007F | +---------------+--------------+--------------+--------------+-----------------+ |110xxxxx |C0-DF 80-BF |300-337 |192-223 |00000080-000007FF| |10xxxxxx | |200-277 |128-191 | | +---------------+--------------+--------------+--------------+-----------------+ |1110xxxx |E0-EF 80-BF |340-357 |224-239 |00000800-0000FFFF| |10xxxxxx |80-BF |200-277 |128-191 | | |10xxxxxx | |200-277 |128-191 | | +---------------+--------------+--------------+--------------+-----------------+ |11110xxx |F0-F7 80-BF |360-367 |240-247 |00010000-001FFFFF| |10xxxxxx |80-BF 80-BF |200-277 |128-191 | | |10xxxxxx | |200-277 |128-191 | | |10xxxxxx | |200-277 |128-191 | | +---------------+--------------+--------------+--------------+-----------------+ |111110xx |F8-FB 80-BF |370-373 |248-251 |00200000-03FFFFFF| |10xxxxxx |80-BF 80-BF |200-277 |128-191 | | |10xxxxxx |80-BF |200-277 |128-191 | | |10xxxxxx | |200-277 |128-191 | | |10xxxxxx | |200-277 |128-191 | | +---------------+--------------+--------------+--------------+-----------------+ |1111110x |FC-FD 80-BF |374-375 |252-253 |04000000-7FFFFFFF| |10xxxxxx |80-BF 80-BF |200-277 |128-191 | | |10xxxxxx |80-BF 80-BF |200-277 |128-191 | | |10xxxxxx | |200-277 |128-191 | | |10xxxxxx | |200-277 |128-191 | | |10xxxxxx | |200-277 |128-191 | | +---------------+--------------+--------------+--------------+-----------------+ I should note that in some cases, you might want to cut slack (or use internally) the hexadecimal sequence C0 80. This is an overlong sequence that, if permitted, can represent ASCII NUL (NIL). Since C and C++ have trouble including a NIL character in an ordinary string, some people have taken to using this sequence when they want to represent NIL as part of the data stream; Java even enshrines the practice. Feel free to use C0 80 internally while processing data, but technically you really should translate this back to 00 before saving the data. Depending on your needs, you might decide to be ``sloppy'' and accept C0 80 as input in a UTF-8 data stream. If it doesn't harm security, it's probably a good practice to accept this sequence since accepting it aids interoperability. ----------------------------------------------------------------------------- 4.8.5. UTF-8 Illegal Values The UTF-8 character set is one case where it's possible to enumerate all illegal values (and prove that you've enumerated them all). You detect an illegal sequence by checking for two things: (1) is the initial sequence legal, and (2) if it is, is the first byte followed by the required number of valid continuation characters? Performing the first check is easy; the following is provably the complete list of all illegal UTF-8 initial sequences: Table 4-2. Illegal UTF-8 initial sequences +-------------------------------------+-------------------------------------+ |UTF-8 Sequence |Reason for Illegality | +-------------------------------------+-------------------------------------+ |10xxxxxx |illegal as initial byte of character | | |(80..BF) | +-------------------------------------+-------------------------------------+ |1100000x |illegal, overlong (C0 80..BF) | +-------------------------------------+-------------------------------------+ |11100000 100xxxxx |illegal, overlong (E0 80..9F) | +-------------------------------------+-------------------------------------+ |11110000 1000xxxx |illegal, overlong (F0 80..8F) | +-------------------------------------+-------------------------------------+ |11111000 10000xxx |illegal, overlong (F8 80..87) | +-------------------------------------+-------------------------------------+ |11111100 100000xx |illegal, overlong (FC 80..83) | +-------------------------------------+-------------------------------------+ |1111111x |illegal; prohibited by spec | +-------------------------------------+-------------------------------------+ The second step is to check if the correct number of continuation characters are included in the string. If the first byte has the top 2 bits set, you count the number of ``one'' bits set after the top one, and then check that there are that many continuation bytes which begin with the bits ``10''. So, binary 11100001 requires two more continuation bytes. Again, although C0 80 is technically an illegal UTF-8 sequence, for interoperability purposes you might want to accept it as a synonym for character 0 (NIL). ----------------------------------------------------------------------------- 4.8.6. UTF-8 Related Issues This section has discussed UTF-8, because it's the most popular multibyte encoding of UCS, simplifying a lot of international text handling issues. However, it's certainly not the only encoding; there are other encodings, such as UTF-16 and UTF-7, which have the same kinds of issues and must be validated for the same reasons. Another issue is that some phrases can be expressed in more than one way in ISO 10646/Unicode. For example, some accented characters can be represented as a single character (with the accent) and also as a set of characters (e.g., the base character plus a separate composing accent). These two forms may appear identical. There's also a zero-width space that could be inserted, with the result that apparently-similar items are considered different. Beware of situations where such hidden text could interfere with the program. This is an issue that in general is hard to solve; most programs don't have such tight control over the clients that they know completely how a particular sequence will be displayed (since this depends on the client's font, display characteristics, locale, and so on). ----------------------------------------------------------------------------- 4.9. Prevent Cross-site Malicious Content on Input Some programs accept data from one untrusted user and pass that data on to a second user; the second user's application may then process that data in a way harmful to the second user. This is a particularly common problem for web applications, we'll call this problem ``cross-site malicious content.'' In short, you cannot accept input (including any form data) without checking, filtering, or encoding it. For more information, see Section 6.13. Fundamentally, this means that all web application input must be filtered (so characters that can cause this problem are removed), encoded (so the characters that can cause this problem are encoded in a way to prevent the problem), or validated (to ensure that only ``safe'' data gets through). Filtering and validation should often be done at the input, but encoding can be done either at input or output time. If you're just passing the data through without analysis, it's probably better to encode the data on input (so it won't be forgotten), but if you're processing the data, there are arguments for encoding on output instead. ----------------------------------------------------------------------------- 4.10. Filter HTML/URIs That May Be Re-presented One special case where cross-site malicious content must be prevented are web applications which are designed to accept HTML or XHTML from one user, and then send it on to other users (see Section 6.13 for more information on cross-site malicious content). The following subsections discuss filtering this specific kind of input, since handling it is such a common requirement. ----------------------------------------------------------------------------- 4.10.1. Remove or Forbid Some HTML Data It's safest to remove all possible (X)HTML tags so they cannot affect anything, and this is relatively easy to do. As noted above, you should already be identifying the list of legal characters, and rejecting or removing those characters that aren't in the list. In this filter, simply don't include the following characters in the list of legal characters: ``< '', ``>'', and ``&'' (and if they're used in attributes, the double-quote character ``"''). If browsers only operated according the HTML specifications, the ``>"'' wouldn't need to be removed, but in practice it must be removed. This is because some browsers assume that the author of the page really meant to put in an opening "<" and ``helpfully'' insert one - attackers can exploit this behavior and use the ">" to create an undesired "< ". Usually the character set for transmitting HTML is ISO-8859-1 (even when sending international text), so the filter should also omit most control characters (linefeed and tab are usually okay) and characters with their high-order bit set. One problem with this approach is that it can really surprise users, especially those entering international text if all international text is quietly removed. If the invalid characters are quietly removed without warning, that data will be irrevocably lost and cannot be reconstructed later. One alternative is forbidding such characters and sending error messages back to users who attempt to use them. This at least warns users, but doesn't give them the functionality they were looking for. Other alternatives are encoding this data or validating this data, which are discussed next. ----------------------------------------------------------------------------- 4.10.2. Encoding HTML Data An alternative that is nearly as safe is to transform the critical characters so they won't have their usual meaning in HTML. This can be done by translating all "<" into "<", ">" into ">", and "&" into "&". Arbitrary international characters can be encoded in Latin-1 using the format "&#value;" - do not forget the ending semicolon. Encoding the international characters means you must know what the input encoding was, of course. One possible danger here is that if these encodings are accidentally interpreted twice, they will become a vulnerability. However, this approach at least permits later users to see the "intent" of the input. ----------------------------------------------------------------------------- 4.10.3. Validating HTML Data Some applications, to work at all, must accept HTML from third parties and send them on to their users. Beware - you are treading dangerous ground at this point; be sure that you really want to do this. Even the idea of accepting HTML from arbitrary places is controversial among some security practitioners, because it is extremely difficult to get it right. However, if your application must accept HTML, and you believe that it's worth the risk, at least identify a list of ``safe'' HTML commands and only permit those commands. Here is a minimal set of safe HTML tags that might be useful for applications (such as guestbooks) that support short comments:

(paragraph), (bold), (italics), (emphasis), (strong emphasis), 

(preformatted text), 
 (forced line break - note it doesn't require a
closing tag), as well as all their ending tags.

Not only do you need to ensure that only a small set of ``safe'' HTML
commands are accepted, you also need to ensure that they are properly nested
and closed. In XML, this is termed ``well-formed'' data. A few exceptions
could be made if you're accepting standard HTML (e.g., supporting an implied

 where not provided before a 
 would be fine), but trying to accept
HTML in its full generality is not needed for most applications. Indeed, if
you're trying to stick to XHTML (instead of HTML), then well-formedness is a
requirement. Also, HTML tags are case-insensitive; tags can be upper case,
lower case, or a mixture. However, if you intend to accept XHTML then you
need to require all tags to be in lower case (XML is case-sensitive; XHTML
uses XML and requires the tags to be in lower case).

Here are a few random tips about doing this. Usually you should design
whatever surrounds the HTML text and the set of permitted tags so that the
contributed text cannot be misinterpreted as text from the ``main'' site (to
prevent forgeries). Don't accept any attributes unless you've checked the
attribute type and its value; there are many attributes that support things
such as Javascript that can cause trouble for your users. You'll notice that
in the above list I didn't include any attributes at all, which is certainly
the safest course. You should probably give a warning message if an unsafe
tag is used, but if that's not practical, encoding the critical characters
(e.g., "<" becomes "<") prevents data loss while simultaneously keeping
the users safe.
-----------------------------------------------------------------------------

4.10.4. Validating Hypertext Links (URIs/URLs)

Careful readers will notice that I did not include the hypertext link tag 
as a safe tag in HTML. Clearly, you could add  (hypertext
link) to the safe list (not permitting any other attributes unless you've
checked their contents). If your application requires it, then do so.
However, permitting third parties to create links is much less safe, because
defining a ``safe URI''[1] turns out to be very difficult. Many browsers
accept all sorts of URIs which may be dangerous to the user. This section
discusses how to validate URIs from third parties for re-presenting to
others, including URIs incorporated into HTML.

First, let's look briefly at URI syntax (as defined by various
specifications). URIs can be either ``absolute'' or ``relative''. The syntax
of an absolute URI looks like this:
scheme://authority[path][?query][#fragment]                                  
A URI starts with a scheme name (such as ``http''), the characters ``://'',
the authority (such as ``www.dwheeler.com''), a path (which looks like a
directory or file name), a question mark followed by a query, and a hash (``#
'') followed by a fragment identifier. The square brackets surround optional
portions - e.g., many URIs don't actually include the query or fragment. Some
schemes may not permit some of the data (e.g., paths, queries, or fragments),
and many schemes have additional requirements unique to them. Many schemes
permit the ``authority'' field to identify optional usernames, passwords, and
ports, using this syntax for the ``authority'' section:
 [username[:password]@]host[:portnumber]                                     
The ``host'' can either be a name (``www.dwheeler.com'') or an IPv4 numeric
address (127.0.0.1). A ``relative'' URI references one object relative to the
``current'' one, and its syntax looks a lot like a filename:
path[?query][#fragment]                                                      
There are a limited number of characters permitted in most of the URI, so to
get around this problem, other 8-bit characters may be ``URL encoded'' as %hh
(where hh is the hexadecimal value of the 8-bit character). For more detailed
information on valid URIs, see IETF RFC 2396 and its related specifications.

Now that we've looked at the syntax of URIs, let's examine the risks of each
part:

  * Scheme: Many schemes are downright dangerous. Permitting someone to
    insert a ``javascript'' scheme into your material would allow them to
    trivially mount denial-of-service attacks (e.g., by repeatedly creating
    windows so the user's machine freezes or becomes unusable). More
    seriously, they might be able to exploit a known vulnerability in the
    javascript implementation. Some schemes can be a nuisance, such as
    ``mailto:'' when a mailing is not expected, and some schemes may not be
    sufficiently secure on the client machine. Thus, it's necessary to limit
    the set of allowed schemes to just a few safe schemes.
   
  * Authority: Ideally, you should limit user links to ``safe'' sites, but
    this is difficult to do in practice. However, you can certainly do
    something about usernames, passwords, and portnumbers: you should forbid
    them. Systems expecting usernames (especially with passwords!) are
    probably guarding more important material; rarely is this needed in
    publically-posted URIs, and someone could try to use this functionality
    to convince users to expose information they have access to and/or use it
    to modify the information. Usernames without passwords are no less
    dangerous, since browsers typically cache the passwords. You should not
    usually permit specification of ports, because different ports expect
    different protocols and the resulting ``protocol confusion'' can produce
    an exploit. For example, on some systems it's possible to use the
    ``gopher'' scheme and specify the SMTP (email) port to cause a user to
    send email of the attacker's choosing. You might permit a few special
    cases (e.g., http ports 8008 and 8080), but on the whole it's not worth
    it. The host when specified by name actually has a fairly limited
    character set (using the DNS standards). Technically, the standard
    doesn't permit the underscore (``_'') character, but Microsoft ignored
    this part of the standard and even requires the use of the underscore in
    some circumstances, so you probably should allow it. Also, there's been a
    great deal of work on supporting international characters in DNS names,
    which is not further discussed here.
   
  * Path: Permitting a path is usually okay, but unfortunately some
    applications use part of the path as query data, creating an opening
    we'll discuss next. Also, paths are allowed to contain phrases like
    ``..'', which can expose private data in a poorly-written web server;
    this is less a problem than it once was and really should be fixed by the
    web server. Since it's only the phrase ``..'' that's special, it's
    reasonable to look at paths (and possibly query data) and forbid ``../''
    as a content. However, if your validator permits URL escapes, this can be
    difficult; now you need to prevent versions where some of these
    characters are escaped, and may also have to deal with various
    ``illegal'' character encodings of these characters as well.
   
  * Query: Query formats (beginning with "?") can be a security risk because
    some query formats actually cause actions to occur on the serving end.
    They shouldn't, and your applications shouldn't, as discussed in Section
    4.11 for more information. However, we have to acknowledge the reality as
    a serious problem. In addition, many web sites are actually
    ``redirectors'' - they take a parameter specifying where the user should
    be redirected, and send back a command redirecting the user to the new
    location. If an attacker references such sites and provides a more
    dangerous URI as the redirection value, and the browser blithely obeys
    the redirection, this could be a problem. Again, the user's browser
    should be more careful, but not all user browsers are sufficiently
    cautious. Also, many web applications have vulnerabilities that can be
    exploited with certain query values, but in general this is hard to
    prevent. The official URI specifications don't sanction the ``+'' (plus)
    character, but in practice the ``+'' character often represents the space
    character.
   
  * Fragment: Fragments basically locate a portion of a document; I'm unaware
    of an attack based on fragments as long as the syntax is legal, but the
    legality of its syntax does need checking. Otherwise, an attacker might
    be able to insert a character such as the double-quote (") and
    prematurely end the URI (foiling any checking).
   
  * URL escapes: URL escapes are useful because they can represent arbitrary
    8-bit characters; they can also be very dangerous for the same reasons.
    In particular, URL escapes can represent control characters, which many
    poorly-written web applications are vulnerable to. In fact, with or
    without URL escapes, many web applications are vulnerable to certain
    characters (such as backslash, ampersand, etc.), but again this is
    difficult to generalize.
   
  * Relative URIs: Relative URIs should be reasonably safe (if you manage the
    web site well), although in some applications there's no good reason to
    allow them either.
   

Of course, there is a trade-off with simplicity as well. Simple patterns are
easier to understand, but they aren't very refined (so they tend to be too
permissive or too restrictive, even more than a refined pattern). Complex
patterns can be more exact, but they are more likely to have errors, require
more performance to use, and can be hard to implement in some circumstances.

Here's my suggestion for a ``simple mostly safe'' URI pattern which is very
simple and can be implemented ``by hand'' or through a regular expression;
permit the following pattern:
(http|ftp|https)://[-A-Za-z0-9._/]+                                          

This pattern doesn't permit many potentially dangerous capabilities such as
queries, fragments, ports, or relative URIs, and it only permits a few
schemes. It prevents the use of the ``%'' character, which is used in URL
escapes and can be used to specify characters that the server may not be
prepared to handle. Since it doesn't permit either ``:'' or URL escapes, it
doesn't permit specifying port numbers, and even using it to redirect to a
more dangerous URI would be difficult (due to the lack of the escape
character). It also prevents the use of a number of other characters; again,
many poorly-designed web applications can't handle a number of ``unexpected''
characters.

Even this ``mostly safe'' URI permits a number of questionable URIs, such as
subdirectories (via ``/'') and attempts to move up directories (via `..'');
illegal queries of this kind should be caught by the server. It permits some
illegal host identifiers (e.g., ``20.20''), though I know of no case where
this would be a security weakness. Some web applications treat subdirectories
as query data (or worse, as command data); this is hard to prevent in general
since finding ``all poorly designed web applications'' is hopeless. You could
prevent the use of all paths, but this would make it impossible to reference
most Internet information. The pattern also allows references to local server
information (through patterns such as "http:///", "http://localhost/", and
"http://127.0.0.1") and access to servers on an internal network; here you'll
have to depend on the servers correctly interpreting the resulting HTTP GET
request as solely a request for information and not a request for an action,
as recommended in Section 4.11. Since query forms aren't permitted by this
pattern, in many environments this should be sufficient.

Unfortunately, the ``mostly safe'' pattern also prevents a number of quite
legitimate and useful URIs. For example, many web sites use the ``?''
character to identify specific documents (e.g., articles on a news site). The
``#'' character is useful for specifying specific sections of a document, and
permitting relative URIs can be handy in a discussion. Various permitted
characters and URL escapes aren't included in the ``mostly safe'' pattern.
For example, without permitting URL escapes, it's difficult to access many
non-English pages. If you truly need such functionality, then you can use
less safe patterns, realizing that you're exposing your users to higher risk
while giving your users greater functionality.

One pattern that permits queries, but at least limits the protocols and ports
used is the following, which I'll call the ``simple somewhat safe pattern'':
 (http|ftp|https)://[-A-Za-z0-9._]+(\/([A-Za-z0-9\-\_\.\!\~\*\'\(\)\%\?]+))*/?
This pattern actually isn't very smart, since it permits illegal escapes,
mutiple queries, queries in ftp, and so on. It does have the advantage of
being relatively simple.

Creating a ``somewhat safe'' pattern that really limits URIs to legal values
is quite difficult. Here's my current attempt to do so, which I call the
``sophisticated somewhat safe pattern'', expressed in a form where whitespace
is ignored and comments are introduced with "#":
 (                                                                              
 (                                                                              
  # Handle http, https, and relative URIs:                                      
  ((https?://([A-Za-z0-9][A-Za-z0-9\-]*(\.[A-Za-z0-9][A-Za-z0-9\-]*)*\.?))|     
    ([A-Za-z0-9\-\_\.\!\~\*\'\(\)]|(%[2-9A-Fa-f][0-9a-fA-F]))+)?                
  ((/([A-Za-z0-9\-\_\.\!\~\*\'\(\)]|(%[2-9A-Fa-f][0-9a-fA-F]))+)*/?) # path     
   (\?(                                                              # query:   
       (([A-Za-z0-9\-\_\.\!\~\*\'\(\)\+]|(%[2-9A-Fa-f][0-9a-fA-F]))+=           
        ([A-Za-z0-9\-\_\.\!\~\*\'\(\)\+]|(%[2-9A-Fa-f][0-9a-fA-F]))+            
        (\&([A-Za-z0-9\-\_\.\!\~\*\'\(\)\+]|(%[2-9A-Fa-f][0-9a-fA-F]))+=        
         ([A-Za-z0-9\-\_\.\!\~\*\'\(\)\+]|(%[2-9A-Fa-f][0-9a-fA-F]))+)*)        
       |                                                                        
       (([A-Za-z0-9\-\_\.\!\~\*\'\(\)\+]|(%[2-9A-Fa-f][0-9a-fA-F]))+  # isindex 
       )                                                                        
   ))?                                                                          
   (\#([A-Za-z0-9\-\_\.\!\~\*\'\(\)\+]|(%[2-9A-Fa-f][0-9a-fA-F]))+)? # fragment 
  )|                                                                            
 # Handle ftp:                                                                  
 (ftp://([A-Za-z0-9][A-Za-z0-9\-]*(\.[A-Za-z0-9][A-Za-z0-9\-]*)*\.?)            
  ((/([A-Za-z0-9\-\_\.\!\~\*\'\(\)]|(%[2-9A-Fa-f][0-9a-fA-F]))+)*/?) # path     
  (\#([A-Za-z0-9\-\_\.\!\~\*\'\(\)\+]|(%[2-9A-Fa-f][0-9a-fA-F]))+)? # fragment  
  )                                                                             
 )                                                                              
 

Even the sophisticated pattern shown above doesn't forbid all illegal URIs.
For example, again, "20.20" isn't a legal domain name, but it's allowed by
the pattern; however, to my knowledge this shouldn't cause any security
problems. The sophisticated pattern forbids URL escapes that represent
control characters (e.g., %00 through $1F) - the smallest permitted escape
value is %20 (ASCII space). Forbidding control characters prevents some
trouble, but it's also limiting; change "2-9" to "0-9" everywhere if you need
to support sending all control characters to arbitrary web applications. This
pattern does permit all other URL escape values in paths, which is useful for
international characters but could cause trouble for a few systems which
can't handle it. The pattern at least prevents spaces, linefeeds,
double-quotes, and other dangerous characters from being in the URI, which
prevents other kinds of attacks when incorporating the URI into a generated
document. Note that the pattern permits ``+'' in many places, since in
practice the plus is often used to replace the space character in queries and
fragments.

Unfortunately, as noted above, there are attacks which can work through any
technique that permit query data, and there don't seem to be really good
defenses for them once you permit queries. So, you could strip out the
ability to use query data from the pattern above, but permit the other forms,
producing a ``sophisticated mostly safe'' pattern:
 (                                                                              
 (                                                                              
  # Handle http, https, and relative URIs:                                      
  ((https?://([A-Za-z0-9][A-Za-z0-9\-]*(\.[A-Za-z0-9][A-Za-z0-9\-]*)*\.?))|     
    ([A-Za-z0-9\-\_\.\!\~\*\'\(\)]|(%[2-9A-Fa-f][0-9a-fA-F]))+)?                
  ((/([A-Za-z0-9\-\_\.\!\~\*\'\(\)]|(%[2-9A-Fa-f][0-9a-fA-F]))+)*/?) # path     
   (\#([A-Za-z0-9\-\_\.\!\~\*\'\(\)\+]|(%[2-9A-Fa-f][0-9a-fA-F]))+)? # fragment 
  )|                                                                            
 # Handle ftp:                                                                  
 (ftp://([A-Za-z0-9][A-Za-z0-9\-]*(\.[A-Za-z0-9][A-Za-z0-9\-]*)*\.?)            
  ((/([A-Za-z0-9\-\_\.\!\~\*\'\(\)]|(%[2-9A-Fa-f][0-9a-fA-F]))+)*/?) # path     
  (\#([A-Za-z0-9\-\_\.\!\~\*\'\(\)\+]|(%[2-9A-Fa-f][0-9a-fA-F]))+)? # fragment  
  )                                                                             
 )                                                                              

As far as I can tell, as long as these patterns are only used to check
hypertext anchors selected by the user (the "" tag) this approach also
prevents the insertion of ``web bugs''. Web bugs are simply text that allow
someone other than the originating web server of the main page to track
information such as who read the content and when they read it - see Section
7.6 for more information. This isn't true if you use the  (image) tag
with the same checking rules - the image tag is loaded immediately,
permitting someone to add a ``web bug''. Once again, this presumes that
you're not permitting any attributes; many attributes can be quite dangerous
and pierce the security you're trying to provide.

Please note that all of these patterns require the entire URI match the
pattern. An unfortunate fact of these patterns is that they limit the
allowable patterns in a way that forbids many useful ones (e.g., they prevent
the use of new URI schemes). Also, none of them can prevent the very real
problem that some web sites perform more than queries when presented with a
query - and some of these web sites are internal to an organization. As a
result, no URI can really be safe until there are no web sites that accept
GET queries as an action (see Section 4.11). For more information about legal
URLs/URIs, see IETF RFC 2396; domain name syntax is further discussed in IETF
RFC 1034.
-----------------------------------------------------------------------------

4.10.5. Other HTML tags

You might even consider supporting more HTML tags. Obvious next choices are
the list-oriented tags, such as 
     (ordered list),