Dulwich: Symlink write-through in checkout(paths=[]) via raw os.open bypasses all symlink protections

What happened?

Summary Dulwich's porcelain.checkout(paths=[...]) code path writes files using raw os.open(file_path, O_WRONLY|O_CREAT|O_TRUNC, mode) followed by f.write(obj.data). This code path does NOT call build_file_from_blob() at all, completely bypassing any symlink protections (including the unreleased d09f8af fix). os.open without O_NOFOLLOW follows symlinks at both the target file and intermediate directories, allowing arbitrary file writes. Root Cause At dulwich/porcelain/init

Key details

• The vulnerable code path writes files directly without calling build_file_from_blob, skipping symlink checks.
• An attacker can craft a malicious repository that writes arbitrary content to any location accessible to the user.
• Writing to .git/hooks/post-checkout enables remote code execution on the next git checkout operation.
• The attack uses a symlink in HEAD pointing to .git/hooks/post-checkout and a malicious executable file in a tag.
• _checked_worktree_path only validates path names and performs zero filesystem symlink detection.
• The suggested fix is to replace raw os.open with build_file_from_blob or add explicit symlink detection using os.path.realpath.

Summarised by SecOraLens from GitHub Security Advisories. Always read the original source before acting. How SecOraLens verifies advisories.