Dulwich: Symlink directory traversal in stash pop allows arbitrary file write via intermediate directory symlinks

Summary

Dulwich's stash.py:pop() function is vulnerable to symlink directory traversal, allowing an attacker to write arbitrary files outside the repository worktree when a victim pops a stash in a malicious repository.

Root Cause

The pop() function at dulwich/stash.py:236 uses os.path.exists(parent_dir) to check if a parent directory exists before writing stashed files. os.path.exists() follows symlinks, so when an intermediate directory in the path is a symlink pointing outside the worktree (e.g., link → ../../.git/hooks), the check passes and subsequent file writes resolve through the symlink.

The validate_path() function (line 228) only validates path component names against INVALID_DOTNAMES — it performs zero filesystem symlink detection. On dulwich 1.2.7 (latest release), build_file_from_blob() has no symlink protection whatsoever.

Impact

An attacker can craft a malicious repository that, when a victim clones it and performs a stash pop operation, writes attacker-controlled content to arbitrary filesystem locations. Writing to .git/hooks/post-checkout achieves Remote Code Execution on the victim's machine on the next git checkout operation.

Attack Scenario

1. Attacker creates a repository with branch main containing link (symlink → ../../.git/hooks) and branch feature containing link/post-checkout (executable payload)
2. Victim clones the repository (landing on main — symlink link exists in worktree)
3. Victim checks out feature, makes changes, runs stash.push()
4. Victim checks out main (restoring the link symlink)
5. Victim runs stash.pop(0) — stash contains link/post-checkout
6. os.path.exists("link") returns True (symlink to existing directory), os.makedirs skipped
7. build_file_from_blob(blob, mode, "link/post-checkout") → open("link/post-checkout", "wb") follows the intermediate symlink → payload written to .git/hooks/post-checkout
8. Next checkout operation triggers the hook → RCE

Suggested Fix

Before writing any file, verify that no component of the target path resolves through a symlink outside the worktree. Use os.path.realpath(parent_dir) and confirm it stays within the repository root. Alternatively, use os.open() with O_NOFOLLOW on each path component.

Reported by zx (Jace)