{"id":"GHSA-8w8g-wq8h-fq33","summary":"Dulwich: Symlink write-through in checkout(paths=[]) via raw os.open bypasses all symlink protections","details":"## 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)**","modified":"2026-10-02T19:00:06.126518030Z","published":"2026-10-02T18:53:35Z","database_specific":{"github_reviewed":true,"github_reviewed_at":"2026-10-02T18:53:35Z","nvd_published_at":null,"cwe_ids":["CWE-59","CWE-61"],"severity":"HIGH"},"references":[{"type":"WEB","url":"https://github.com/jelmer/dulwich/security/advisories/GHSA-8w8g-wq8h-fq33"},{"type":"WEB","url":"https://github.com/jelmer/dulwich/commit/9389fcb5cb9113adfc7f207d8be86a56904db3e8"},{"type":"PACKAGE","url":"https://github.com/jelmer/dulwich"},{"type":"WEB","url":"https://github.com/jelmer/dulwich/releases/tag/dulwich-1.2.8"}],"affected":[{"package":{"name":"dulwich","ecosystem":"PyPI","purl":"pkg:pypi/dulwich"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0.24.0"},{"fixed":"1.2.8"}]}],"versions":["0.24.0","0.24.1","0.24.10","0.24.2","0.24.3","0.24.4","0.24.5","0.24.6","0.24.7","0.24.8","0.24.9","0.25.0","0.25.1","0.25.2","1.0.0","1.1.0","1.2.0","1.2.1","1.2.2","1.2.3","1.2.4","1.2.5","1.2.6","1.2.7"],"database_specific":{"source":"https://github.com/github/advisory-database/blob/main/advisories/github-reviewed/2026/10/GHSA-8w8g-wq8h-fq33/GHSA-8w8g-wq8h-fq33.json","last_known_affected_version_range":"\u003c= 1.2.7"}}],"schema_version":"1.9.0","severity":[{"type":"CVSS_V3","score":"CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:C/C:H/I:H/A:H"}]}