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

The vulnerability arises because Dulwich's porcelain.checkout(paths=[...]) implementation writes files using raw os.open without O_NOFOLLOW, which follows symlinks at both the target and intermediate directories. The path validation only checks for invalid names but does not detect symlinks. This allows an attacker to craft a repository where a symlink points to sensitive locations like .git/hooks/post-checkout. When a victim runs checkout with that path, the malicious payload is written to the hook file, leading to remote code execution on the next checkout. The suggested fix is to replace the raw os.open call with a hardened build_file_from_blob function or add explicit symlink detection to ensure the path stays within the worktree root.

An attacker can cause arbitrary file writes with attacker-controlled content and permissions to any location accessible by the user running the checkout. This can lead to remote code execution by overwriting git hook scripts such as .git/hooks/post-checkout. The vulnerability compromises confidentiality, integrity, and availability of the affected system.

A patch is available for this vulnerability. Users should upgrade Poetry to a fixed version at or beyond 2.4.2. Until patched, avoid running checkout(paths=[...]) on untrusted repositories. The vendor advisory recommends replacing raw os.open usage with a hardened function that detects symlinks or adding explicit symlink detection and path resolution to ensure files are written only within the worktree.