GHSA-fjr4-x663-mwxc

## 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 ```python # 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 (`ot