How Can I Safely Pass Variables to `subprocess.Popen()` Without Security Risks?

Overcoming the Pitfalls of Variable Passing in Subprocess.Popen()

In many scripts, the need to call external programs with arguments arises. When these arguments are stored in variables, issues can arise with subprocess.Popen(). While it may seem straightforward, this process can be hindered by certain pitfalls.

One such pitfall occurs when using shell=True. This option treats the arguments differently on Unix systems, leading to unexpected behavior. To navigate this challenge, it is recommended to drop shell=True.

Example:

Consider the following scenario:

import subprocess

# Populate a list of arguments
args = ["mytool.py"]
for opt, optname in zip("-a -x -p".split(), "address port pass".split()):
    args.extend([opt, str(servers[server][optname])])
args.extend("some additional command".split())

# Run the script without shell=True
p = subprocess.Popen([sys.executable or 'python'] + args, stdout=subprocess.PIPE)

With this approach, the arguments are handled accurately, allowing the external program to receive the intended inputs.

Security Considerations:

It is important to note that setting shell=True for commands with external input poses a security hazard. As explained in the subprocess documentation, this practice can expose your script to potential vulnerabilities.

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