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• Use full paths for the programs you are executing. For example,
  • shell: /bin/ls -l file
  • python: subprocess.Popen(["/bin/ls", "-l", "file"])
• If your external programs can be found in a variety of locations depending on the system on which they are run, use exact control over the PATH used to find your programs. For example, you can add a line to the beginning of your program such as:
  
  • shell: export PATH=/bin:/usr/bin or export PATH=/sbin:/usr/sbin:/bin:/usr/bin
  • python: os.environ['PATH'] = '/bin:/usr/bin' or os.environ['PATH'] = '/sbin:/usr/sbin:/bin:/usr/bin'
• If you must depend on an externally-provided path, combine prepending known locations to the beginning of the PATH, and sanitize with sanitizing the rest of it. For example, you can add a line to the beginning of your program such as:
  • bash shell: export PATH="/bin:/usr/bin:$(sanitize "$PATH")"
  • python: os.environ['PATH'] = f"/bin:/usr/bin:{sanitize(os.environ['PATH'])}"
  • The purpose of You will have to write the sanitize() invocation function. Some things to consider are:
    
    • The current directory is dangerous in the PATH, except possibly at the end. So one thing your sanitize function should do is to remove
    the current directory from the PATH. For example, TBA
    • the current directory, or any aliases that map into the current directory (e.g. ::, :.:, :./:, :./.:, etc.).
    • It's also dangerous for any of the directories found in the PATH to be world writable, where someone can create a program with the same name as a system tool invoked by your program. (Consider having a PATH with /tmp in it, where someone could have placed a script named "ls".)

• Use exact control over the command line arguments. For example, in Python avoid using "shell=True" with the subprocess module's methods, which can allow unexpected parsing or expansion of the values being passed.

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