GitPython blind local file inclusion

QFinch assessment

In order to resolve some git references, GitPython reads files from the .git directory, in some places the name of the file being read is provided by the user, GitPython doesn't check if this file is located outside the .git directory. This allows an attacker to make GitPython read any file from the system.

Potential impact

I wasn't able to show the contents of the files (that's why "blind" local file inclusion), depending on how GitPython is being used, this can be used by an attacker for something inoffensive as checking if a file exits, or cause a DoS by making GitPython read a big/infinite file (like /dev/random on Linux systems).

Technical context

This vulnerability is present in https://github.com/gitpython-developers/GitPython/blob/1c8310d7cae144f74a671cbe17e51f63a830adbf/git/refs/symbolic.pyL174-L175 That code joins the base directory with a user given string without checking if the final path is located outside the base directory. I was able to exploit it from three places, but there may be more code paths that lead to it: https://github.com/gitpython-developers/GitPython/blob/1c8310d7cae144f74a671cbe17e51f63a830adbf/git/repo/base.pyL605 https://github.com/gitpython-developers/GitPython/blob/1c8310d7cae144f74a671cbe17e51f63a830adbf/git/repo/base.pyL620 https://github.com/gitpython-developers/GitPython/blob/1c8310d7cae144f74a671cbe17e51f63a830adbf/git/index/base.pyL1353