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Next Previous Contents Secure Programming for Linux and Unix HOWTO David A. Wheeler, dwheeler@dwheeler.com version 1.60, 4 April 2025 This paper 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, CGI scripts, network servers, and setuid/setgid programs. 1. Introduction 2. Background 2.1 History of Unix, Linux, and Open Source Software 2.2 Security Principles 2.3 Types of Secure Programs 2.4 Paranoia is a Virtue 2.5 Why Did I Write This Document? 2.6 Sources of Design and Implementation Guidelines 2.7 Document Conventions 3. Summary of Linux and Unix Security Features 3.1 Processes 3.2 Files 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 4. Validate All Input 4.1 Command line 4.2 Environment Variables 4.3 File Descriptors 4.4 File Contents 4.5 CGI Inputs 4.6 Other Inputs 4.7 Character Encoding 4.8 Limit Valid Input Time and Load Level 5. Avoid Buffer Overflow 5.1 Dangers in C/C++ 5.2 Library Solutions in C/C++ 5.3 Compilation Solutions in C/C++ 5.4 Other Languages 6. Structure Program Internals and Approach 6.1 Secure the Interface 6.2 Minimize Privileges 6.3 Configure Safely and Use Safe Defaults 6.4 Fail Safe 6.5 Avoid Race Conditions 6.6 Trust Only Trustworthy Channels 6.7 Use Internal Consistency-Checking Code 6.8 Self-limit Resources 7. Carefully Call Out to Other Resources 7.1 Limit Call-outs to Valid Values 7.2 Check All System Call Returns 8. Send Information Back Judiciously 8.1 Minimize Feedback 8.2 Handle Full/Unresponsive Output 9. Special Topics 9.1 Locking 9.2 Passwords 9.3 Random Numbers 9.4 Cryptographic Algorithms and Protocols 9.5 Java 9.6 PAM 9.7 Miscellaneous 10. Conclusion 11. References 12. Credits 13. Document License Next Previous Contents