GHSA-cm62-gvxx-vmxx: Dulwich Path Traversal Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-cm62-gvxx-vmxx",
  "modified" : "2026-10-02T18:52:53Z",
  "published" : "2026-10-02T18:52:53Z",
  "aliases" : [ ],
  "summary" : "Dulwich: Symlink directory traversal in stash pop allows arbitrary file write via intermediate directory symlinks",
  "details" : "## Summary\n\nDulwich's `stash.py:pop()` function is vulnerable to symlink directory traversal, allowing an attacker to write arbitrary files outside the repository worktree when a victim pops a stash in a malicious repository.\n\n## Root Cause\n\nThe `pop()` function at `dulwich/stash.py:236` uses `os.path.exists(parent_dir)` to check if a parent directory exists before writing stashed files. `os.path.exists()` follows symlinks, so when an intermediate directory in the path is a symlink pointing outside the worktree (e.g., `link → ../../.git/hooks`), the check passes and subsequent file writes resolve through the symlink.\n\nThe `validate_path()` function (line 228) only validates path component names against `INVALID_DOTNAMES` — it performs zero filesystem symlink detection. On dulwich 1.2.7 (latest release), `build_file_from_blob()` has no symlink protection whatsoever.\n\n## Impact\n\nAn attacker can craft a malicious repository that, when a victim clones it and performs a stash pop operation, writes attacker-controlled content to arbitrary filesystem locations. Writing to `.git/hooks/post-checkout` achieves Remote Code Execution on the victim's machine on the next git checkout operation.\n\n## Attack Scenario\n\n1. Attacker creates a repository with branch `main` containing `link` (symlink → `../../.git/hooks`) and branch `feature` containing `link/post-checkout` (executable payload)\n2. Victim clones the repository (landing on `main` — symlink `link` exists in worktree)\n3. Victim checks out `feature`, makes changes, runs `stash.push()`\n4. Victim checks out `main` (restoring the `link` symlink)\n5. Victim runs `stash.pop(0)` — stash contains `link/post-checkout`\n6. `os.path.exists(\"link\")` returns True (symlink to existing directory), `os.makedirs` skipped\n7. `build_file_from_blob(blob, mode, \"link/post-checkout\")` → `open(\"link/post-checkout\", \"wb\")` follows the intermediate symlink → payload written to `.git/hooks/post-checkout`\n8. Next checkout operation triggers the hook → RCE\n\n## Suggested Fix\n\nBefore writing any file, verify that no component of the target path resolves through a symlink outside the worktree. Use `os.path.realpath(parent_dir)` and confirm it stays within the repository root. Alternatively, use `os.open()` with `O_NOFOLLOW` on each path component.\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" : "0.22.5"
      }, {
        "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-cm62-gvxx-vmxx"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/jelmer/dulwich/commit/40a542cb02f9ac39a9eeac1193472903098698f9"
  }, {
    "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-22", "CWE-59" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-10-02T18:52:53Z",
    "nvd_published_at" : null
  }
}