GitPython: Incomplete unsafe_git_clone_options denylist omits --template enabling arbitrary command execution via clone hooks

Summary

GitPython's unsafe_git_clone_options denylist omits --template. git clone --template=<dir> copies <dir>/hooks/ into the new repository and runs them (post-checkout fires during clone), so a caller who can influence clone options can achieve arbitrary command execution in the default allow_unsafe_options=False configuration.

Root Cause

base.py:145-152 defines unsafe_git_clone_options = ["--upload-pack","-u","--config","-c"] — --template is absent. The guard candidate ['--template'] passes check_unsafe_options (verified). git copies the hook directory and executes post-checkout at checkout time. git's protocol.allow/GIT_ALLOW_PROTOCOL do not gate --template; the incomplete denylist is the only defense.

Impact

Arbitrary OS command execution during clone (default config). Requires an attacker-readable directory containing an executable hook — a genuine second precondition (realistic via shared filesystems, upload dirs, /tmp, or attacker-writable network paths), reflected as AC:H.

Proof of Concept

# attacker stages <dir>/hooks/post-checkout (chmod +x)
from git import Repo
Repo.clone_from(src, dst, template='<dir>')   # post-checkout hook executes -> marker created (verified)

Attack Chain

1. Setup: attacker stages <dir>/hooks/post-checkout (chmod +x). Guard: n/a (filesystem).
2. Entry: Repo.clone_from(url, path, template='<dir>'). Guard: check_unsafe_options(candidates=['--template'], unsafe=unsafe_git_clone_options). Bypass proof: --template not on the denylist -> passes (verified candidate ['--template'], no error).
3. Sink: git copies the hook and executes post-checkout at checkout. Impact: ACE, default config (verified marker created).

Bypass Evidence

Live-verified on HEAD (tag 3.1.53): guard candidate ['--template'] passed with no error; staged post-checkout hook executed during clone_from, creating the marker. Independent of the value-smuggle bypass (--template is a legitimate long option that survives any single-char-value fix). Not covered by any existing advisory.

Affected Versions

<= 3.1.53