{"id":"CVE-2026-114096","aliases":["GHSA-8w8g-wq8h-fq33"],"url":"https://o3.security/vulnerability/CVE-2026-114096","summary":"Dulwich 0.24.0 before 1.2.8 Symlink Write-Through via porcelain.checkout paths","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)**","published":"2026-10-21T00:36:59.763Z","modified":"2026-10-24T02:49:31.336147517Z","cvss":null,"epss":null,"cisaKev":null,"exploitsKnown":null,"affectedPackages":[{"ecosystem":"PyPI","name":"dulwich","fixedVersion":"1.2.8"}],"fix":{"url":"https://github.com/jelmer/dulwich/commit/9389fcb5cb9113adfc7f207d8be86a56904db3e8","label":"jelmer/dulwich@9389fcb"},"references":[{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/114xxx/CVE-2026-114096.json"},{"type":"ADVISORY","url":"https://github.com/jelmer/dulwich/security/advisories/GHSA-8w8g-wq8h-fq33"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-114096"},{"type":"ADVISORY","url":"https://www.vulncheck.com/advisories/dulwich-0.24.0-before-1.2.8-symlink-write-through-via-porcelain-checkout-paths"},{"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"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-10-24T02:49:31.336147517Z"}}