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12 Principles of Secure Linux Programming

1. Operate with least privilege

  • Hold privileges only while they're required.
  • Drop privileges permanently when they'll never be used again.
  • A privileged program should never exec a shell unless the environment can silently ignore set-user-ID and set-group-ID permissions.
  • Close all unnecessary file descriptors before an exec.

2. Avoid exposing sensitive information

  • Use mlock to lock access to the virtual memory page in use.
  • Prevent core dumps by using setrlimit.

3. Confine the process

  • Use capabilities and securebit flags whenever possible.
  • Establish a chroot jail to limit the set of directories and files that a program may access.
  • Use a virtual server: UML, Xen, KVM, etc.

4. Beware of signals and race conditions

  • Signals should be caught, blocked, or ignored to prevent possible security problems.

5. Pitfalls of file operations and file I/O

  • Umask should be set to a value that ensures the process never creates publicly writable files.
  • Use seteuid and setreuid to temporarily change process credentials to ensure new files do not belong to the wrong user.
  • Never create a file owned by the program owner.
  • Never allow other users to write to files that the process uses.
  • Use mkstemp to create files with unpredictable names.

6. Don't trust inputs or the environment

  • Do not assume values of environment variables are reliable.
  • Validate all inputs from untrusted sources.
  • Avoid unreliable assumptions about the process's run-time environment.

7. Beware of injection attacks

  • Use regex pattern matching to filter user inputs.
  • Encode user input into Base-64 encoded strings.
  • Never compare raw strings when checking credentials.

8. Beware of buffer overruns

  • Do not allow an input value or a copied string to exceed allocated buffer space.
  • Use if statements that prevent buffer overruns.

9. Beware of denial-of-service attacks

  • Minimize the risk and consequences of overload attacks.
    • Perform load throttling.
    • Use resource limits and disk quotas.
  • Employ timeouts for communication with clients.
  • Perform log throttling.
  • Perform bounds checking on data structures.
  • Design data structures that avoid algorithmic-complexity attacks.

10. Beware of database attacks

  • Always use a mutex to ensure the right process has access at the right time.
  • Use databases that enforce host-based security policies.
  • Use databases that enforce separation of database ownership and table ownership.

11. Check return statuses and fail safely

  • Always check the program's return values.
  • Store the value then check it before returning it.
  • Return it then ask for verification.
  • Unexpected situations must cause the program to terminate or drop the client request.

12. Beware of reverse-engineering attacks

  • Avoid hardcoding sensitive information in string literals.
  • Generate one-time IDs for runtime execution.
  • Salt IDs with unique hardware device addresses.

Do you have a suggestion about how to improve this blog? Let's talk about it. Contact me at David.Brenner.Jr@Gmail.com or 720-584-5229.

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