GHSA-cwvm-v4w8-q58c

Description

### Summary 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. ### Details This vulnerability is present in https://github.com/gitpython-developers/GitPython/blob/1c8310d7cae144f74a671cbe17e51f63a830adbf/git/refs/symbolic.py#L174-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.py#L605 https://github.com/gitpython-developers/GitPython/blob/1c8310d7cae144f74a671cbe17e51f63a830adbf/git/repo/base.py#L620 https://github.com/gitpython-developers/GitPython/blob/1c8310d7cae144f74a671cbe17e51f63a830adbf/git/index/base.py#L1353 ### PoC Running GitPython within any repo should work, here is an example with the GitPython repo. ```python import git r = git.Repo(".") # This will make GitPython read the README.md file from the root of the repo r.commit("../README.md") r.tree("../README.md") r.index.diff("../README.md") # Reading /etc/random # WARNING: this will probably halt your system, run with caution # r.commit("../../../../../../../../../dev/random") ``` ### 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). ### Possible solutions A solution would be to check that the final path isn't located outside the `repodir` path (maybe even after resolving symlinks). Maybe there could be other checks in place to make sure that the reference names are valid.