GitPython: Arbitrary file overwrite via git diff --output argument injection in Diffable.diff (key- and value-controlled)

Summary

Diffable.diff() forwards **kwargs straight into diff/diff_tree with no check_unsafe_options guard. Diffable is mixed into Commit, Tree, IndexFile, and Submodule, giving a broad surface. git diff --output=<path> writes real patch content to an attacker-chosen path, enabling arbitrary file overwrite.

Root Cause

diff.py:188-283 builds and runs the diff command with no check_unsafe_options anywhere in the method (grep-confirmed). Additionally diff.py:265 does args.insert(0, other), placing the caller-supplied other ref BEFORE the -- separator, so a value of --output=/path is parsed by git as an option — a value-only control path requiring no kwarg key.

Impact

Overwrite/corrupt any file at process privilege with attacker-chosen path (e.g. ~/.ssh/authorized_keys, configs, lockfiles). Content is real diff/patch bytes (attacker-influenced). Per the skill's rule, controlling WHICH file is overwritten = I:H regardless of content constraints.

Proof of Concept

# Key-control:
commit.diff(other_commit, output='/home/app/.ssh/authorized_keys')   # victim overwritten with diff (105 bytes verified)
# Value-control (attacker controls only the ref string):
commit.diff(other='--output=/home/app/.ssh/authorized_keys')          # 14-byte victim -> 146 bytes of diff-tree output

Attack Chain

1. Entry (value-control): commit.diff(other=<user ref>) with other = "--output=/home/app/.ssh/authorized_keys". Guard: none in Diffable.diff. Bypass proof: no check_unsafe_options in the method body (grep); other inserted pre--- at diff.py:265.
2. Sink: git diff-tree <sha> --output=/home/app/.ssh/authorized_keys -r ... -> git opens+truncates the target then writes diff content. Impact: overwrite/corrupt any file at process privilege (attacker chooses the path). Verified argv and victim overwrite live.

Bypass Evidence

Live-verified on HEAD (tag 3.1.53): both key-control (output=) and value-control (other='--output=...') overwrote a victim file with real diff-tree content; argv confirmed ['git','diff-tree','<sha>','--output=/victim','-r',...]. This is the same value-control model GHSA-956x deemed fix-worthy for — but is a distinct, unguarded sink NOT touched by that fix.