{"id":"GHSA-8w8g-wq8h-fq33","title":"Dulwich: Symlink write-through in checkout(paths=[]) via raw os.open bypasses all symlink protections","summary":"Dulwich: Symlink write-through in checkout(paths=[]) via raw os.open bypasses all symlink protections","severity":"high","cvss":8.6,"cwe":["CWE-59","CWE-61"],"vendor":"dulwich","product":"dulwich","ecosystem":"pip","affected":["dulwich >= 0.24.0, <= 1.2.7"],"patched":["dulwich 1.2.8"],"published":"2026-10-02","updated":"2026-10-02","sourceUpdated":"2026-10-02T18:53:36Z","source":"GHSA","sourceUrl":"https://github.com/advisories/GHSA-8w8g-wq8h-fq33","references":[{"url":"https://github.com/jelmer/dulwich/security/advisories/GHSA-8w8g-wq8h-fq33"},{"url":"https://github.com/jelmer/dulwich/commit/9389fcb5cb9113adfc7f207d8be86a56904db3e8"},{"url":"https://github.com/jelmer/dulwich/releases/tag/dulwich-1.2.8"},{"url":"https://github.com/advisories/GHSA-8w8g-wq8h-fq33"}],"tags":["ghsa","pip"],"ingestedAt":"2026-10-02T22:33:09.856Z","slug":"GHSA-8w8g-wq8h-fq33","body":"## Overview\n\n## Summary\n\nDulwich'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.\n\n## Root Cause\n\nAt `dulwich/porcelain/__init__.py:5661-5675`, the `checkout(paths=[...])` implementation:\n\n```python\nfile_path = _checked_worktree_path(r, path)\nos.makedirs(os.path.dirname(file_path), exist_ok=True)\nflags = os.O_WRONLY | os.O_CREAT | os.O_TRUNC\nwith os.fdopen(os.open(file_path, flags, mode), \"wb\") as f:\n    f.write(obj.data)\n```\n\n`_checked_worktree_path()` (line 601-631) only performs name validation — checking that the path doesn't start with `/` or `\\\\` and that components pass `INVALID_DOTNAMES` checks. It performs zero filesystem symlink detection.\n\n## Impact\n\nAn attacker can craft a malicious repository that, when a victim clones it and runs `checkout(paths=[...])`, writes attacker-controlled content (with attacker-controlled permissions) to any filesystem location accessible to the user. Writing to `.git/hooks/post-checkout` achieves RCE on the next git checkout.\n\n## Attack Scenario\n\n1. Attacker creates a repository where HEAD has `trigger` as a symlink (mode 120000, content `../../.git/hooks/post-checkout`), and tag `v1.0` has `trigger` as an executable file (mode 100755, content `#!/bin/sh\\nmalicious_payload`)\n2. Victim clones the repository — worktree has `trigger` → `../../.git/hooks/post-checkout` (a symlink)\n3. Victim runs `porcelain.checkout(repo, target=\"v1.0\", paths=[\"trigger\"])` to restore a specific file from a tag\n4. `_checked_worktree_path(r, \"trigger\")` passes — name validation only, no symlink check\n5. `os.open(\"trigger\", O_WRONLY|O_CREAT|O_TRUNC, 0o755)` follows the symlink → opens `.git/hooks/post-checkout` for writing\n6. `f.write(obj.data)` writes the malicious payload to the hook\n7. Next checkout operation triggers the hook → RCE\n\n## Suggested Fix\n\nReplace the raw `os.open` path with a call to `build_file_from_blob` (once that function is hardened against intermediate symlinks), or add explicit symlink detection: resolve the path with `os.path.realpath()` and verify it stays within the worktree root before opening.\n\nReported by **zx (Jace)**\n\n## Affected packages\n\n- `dulwich >= 0.24.0, <= 1.2.7`\n\n## Remediation\n\nUpgrade to a patched release:\n\n- `dulwich 1.2.8`","depth":"twilight","depthScore":47,"depthScoreParts":{"impact":47.3,"likelihood":0,"exploitation":0,"ransomware":0},"changes":[]}