GitPython untrusted search path on Windows systems leading to arbitrary code execution

Summary

When resolving a program, Python/Windows look for the current working directory, and after that the PATH environment (see big warning in https://docs.python.org/3/library/subprocess.html#popen-constructor). GitPython defaults to use the git command, if a user runs GitPython from a repo has a git.exe or git executable, that program will be run instead of the one in the user's PATH.

Details

This is more of a problem on how Python interacts with Windows systems, Linux and any other OS aren't affected by this. But probably people using GitPython usually run it from the CWD of a repo.

The execution of the git command happens in

https://github.com/gitpython-developers/GitPython/blob/1c8310d7cae144f74a671cbe17e51f63a830adbf/git/cmd.py#L277

https://github.com/gitpython-developers/GitPython/blob/1c8310d7cae144f74a671cbe17e51f63a830adbf/git/cmd.py#L983-L996

And there are other commands executed that should probably be aware of this problem.

PoC

On a Windows system, create a git.exe or git executable in any directory, and import or run GitPython from that directory

python -c "import git"

The git executable from the current directory will be run.

Impact

An attacker can trick a user to download a repository with a malicious git executable, if the user runs/imports GitPython from that directory, it allows the attacker to run any arbitrary commands.

Possible solutions

• Default to an absolute path for the git program on Windows, like C:\\Program Files\\Git\\cmd\\git.EXE (default git path installation).
• Require users to set the GIT_PYTHON_GIT_EXECUTABLE environment variable on Windows systems.
• Make this problem prominent in the documentation and advise users to never run GitPython from an untrusted repo, or set the GIT_PYTHON_GIT_EXECUTABLE env var to an absolute path.
• Resolve the executable manually by only looking into the PATH environment variable (suggested by @Byron)

[!NOTE] This vulnerability was reported via email, and it was decided to publish it here and make it public, so the community is aware of it, and a fix can be provided.