GHSA-8w8g-wq8h-fq33: Dulwich Symlink Write-through Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-8w8g-wq8h-fq33",
  "modified" : "2026-10-02T18:53:35Z",
  "published" : "2026-10-02T18:53:35Z",
  "aliases" : [ ],
  "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)**",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:C/C:H/I:H/A:H"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "PyPI",
      "name" : "dulwich"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0.24.0"
      }, {
        "fixed" : "1.2.8"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 1.2.7"
    }
  } ],
  "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"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-59", "CWE-61" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-10-02T18:53:35Z",
    "nvd_published_at" : null
  }
}