The Diffable.diff() method in Tach's GitPython integration forwards **kwargs directly into git diff/diff-tree commands without invoking check_unsafe_options, allowing injection of git command-line options. Specifically, the 'other' argument is inserted before the '--' separator, enabling an attacker to supply '--output=<path>' which causes git to write diff output to an arbitrary file path. This leads to arbitrary file overwrite at the process privilege level. Both key-controlled (e.g., output='/path') and value-controlled (e.g., other='--output=/path') injections were verified on version 3.1.53. The vulnerability affects all versions up to and including 3.1.53. The suggested fix is to add check_unsafe_options to Diffable.diff and/or place a '--end-of-options' marker before the 'other' argument to prevent option parsing.

An attacker with the ability to call Diffable.diff() with crafted arguments can overwrite or corrupt any file writable by the process, including sensitive files such as ~/.ssh/authorized_keys, configuration files, or lockfiles. The content written is the real diff or patch output generated by git, which is attacker-influenced. This can lead to privilege escalation or persistent compromise depending on the overwritten file. The impact is high due to arbitrary file overwrite at process privilege.

A patch is available that adds the check_unsafe_options guard to Diffable.diff, preventing injection of unsafe git command options. Additionally, placing a '--end-of-options' marker before the 'other' argument prevents it from being interpreted as an option. Users should upgrade to a fixed version beyond 3.1.53 or apply the patch if available. Until patched, avoid passing untrusted input to Diffable.diff's arguments that could be interpreted as git options.