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

The vulnerability exists in Dulwich's stash.py pop() function where it checks for parent directory existence using os.path.exists(), which follows symlinks. This allows an attacker to create a malicious repository with intermediate symlinks pointing outside the worktree. When a victim performs stash pop, files can be written outside the repository, such as to .git/hooks/post-checkout, enabling remote code execution. The validate_path() function does not detect symlinks, and build_file_from_blob() lacks symlink protection. The suggested fix involves verifying that no path component resolves outside the repository root using os.path.realpath() or using os.open() with O_NOFOLLOW.

An attacker can cause arbitrary file writes outside the repository worktree on a victim's machine by exploiting symlink traversal during stash pop operations. This can lead to remote code execution by writing malicious git hook scripts, such as .git/hooks/post-checkout, which execute on subsequent git checkouts. The vulnerability affects confidentiality, integrity, and availability of the victim's system.

A patch is available for this vulnerability. Users should upgrade to a fixed version of Dulwich that includes checks to ensure no path component resolves through symlinks outside the repository root before writing files. The fix involves validating paths using os.path.realpath() or opening files with O_NOFOLLOW to prevent symlink traversal. Until patched, avoid popping stashes from untrusted repositories.