PYSEC-2026-3837

Summary

TagReference.create() forwards a caller-influenced positional reference value into git tag without it ever being inspected by the unsafe-option guard, allowing an arbitrary file read (the file's contents are returned in-band as the annotated tag message). This is an incomplete-fix bypass of commit 3af0c251 (the fix for GHSA-3f7w-8rr8-f37f's tag instance).

Root Cause

The fix 3af0c251 added unsafe_git_tag_options = ["--file","-F"] and a guard call, but the guard is Git.check_unsafe_options(options=Git._option_candidates([], kwargs), unsafe_options=...) at git/refs/tag.py:139 — it passes an EMPTY args list and inspects kwargs only. The dangerous values path and reference are POSITIONALS (args = (path, reference), tag.py:156), placed before any --. A user-influenced reference="--file=<path>" therefore reaches git tag as the exact --file option the fix intended to block, creating an annotated tag whose message is the file's contents.

Impact

Arbitrary local file read at the privileges of the host process; contents returned in-band via tagref.tag.message. Requires the embedding application to forward a caller-influenced reference value into TagReference.create() (pure VALUE control — the CVE-2026-42215 threat model). Default allow_unsafe_options=False.

Proof of Concept

from git import TagReference
t = TagReference.create(repo, "vpwn", reference="--file=/home/app/.ssh/id_rsa")
print(t.tag.message)   # contents of the file

Attack Chain

1. Entry: app calls TagReference.create(repo, name, reference=<user>) with reference="--file=/home/app/.ssh/id_rsa".
2. Check: Git.check_unsafe_options(_option_candidates([], kwargs), ["--file","-F"]) @ tag.py:137-141. Guard: denylist includes --file/-F. Bypass proof: _option_candidates receives args=[] → the positional reference is never a candidate (the kwarg spelling file="…" IS blocked; only the positional escapes).
3. Sink: repo.git.tag(*args, **kwargs) @ tag.py:158 → no --. argv (observed): ['git','tag','-f','vpwn','--file=<secret>'].
4. Impact: annotated tag created; tagref.tag.message == file contents (arbitrary file read).

Bypass Evidence

Independently reproduced (independent test harness, git 2.43.0, default allow_unsafe_options=False): TagReference.create(repo,'vp','--file=<secret>') → PASSED; tag.message == 'GATE_SECRET_LINE_A\nGATE_SECRET_LINE_B'. Control: TagReference.create(..., file='<secret>') → UnsafeOptionError: --file is not allowed. Fix-commit read: 3af0c251 adds _option_candidates([], kwargs) (empty args → positional never a candidate).

Affected Versions

GitPython <= 3.1.58 (sink present verbatim on the latest release tag; git diff 3.1.57..HEAD touches only test files).

Suggested Fix

Include the positional reference (and path) in the option-candidate list passed to check_unsafe_options, or place a -- separator before the positional arguments in TagReference.create().

Reported by zx (Jace) — GitHub: @manus-use