GHSA-5fqc-mrg8-w798: Dulwich Path Traversal Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-5fqc-mrg8-w798",
  "modified" : "2026-10-02T18:53:05Z",
  "published" : "2026-10-02T18:53:05Z",
  "aliases" : [ ],
  "summary" : "Dulwich: Symlink directory traversal in filter-branch index_filter via cross-commit state persistence",
  "details" : "## Summary\n\nDulwich's `filter_branch.py` `CommitFilter._apply_index_filter()` is vulnerable to symlink directory traversal. When processing commit history, materialized tree entries (including symlinks) persist in the working directory between commits, allowing a symlink from an ancestor commit to redirect file writes from a descendant commit to arbitrary filesystem locations.\n\n## Root Cause\n\n`_apply_index_filter()` at `dulwich/filter_branch.py:212` calls `build_index_from_tree(\".\", tmp_index_path, ...)` which materializes all tree entries to the current working directory. The `finally` block (line 229-230) only cleans up the temporary index file (`os.unlink(tmp_index_path)`) — NOT the filesystem files written to CWD. When `process_commit()` processes parents recursively first (line 260), files materialized from ancestor commits persist and affect processing of descendant commits.\n\nOn dulwich 1.2.7, `build_file_from_blob()` has no symlink protection, and `validate_path_element` only validates name patterns, not filesystem state.\n\n## Impact\n\nAn attacker can craft a malicious repository where running `filter_branch` with an index filter writes attacker-controlled content to arbitrary filesystem locations via symlink traversal. This achieves RCE if the write targets `.git/hooks/`.\n\n## Attack Scenario\n\n1. Attacker creates a repository where commit history (linearized) has:\n   - Ancestor commit: tree entry `evil` (mode 120000, symlink → `/target_dir`)\n   - Descendant commit: tree entry `evil/payload` (mode 100644, attacker content)\n2. Victim clones repository and runs `filter_branch` with an index filter\n3. `process_commit()` processes ancestor first → materializes `evil` as symlink to `/target_dir` in CWD\n4. CWD is NOT cleaned between commits\n5. Processing descendant: `os.path.exists(\"./evil\")` → True (symlink exists). `build_file_from_blob(blob, mode, \"./evil/payload\")` → `open(\"./evil/payload\", \"wb\")` follows intermediate symlink → writes to `/target_dir/payload`\n\n## Suggested Fix\n\nClean the CWD between commit iterations in `_apply_index_filter()`, or verify that no intermediate path components are symlinks before writing files.\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" : "0.23.1"
      }, {
        "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-5fqc-mrg8-w798"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/jelmer/dulwich/commit/9571ac60b851fce228dae7ededb380c6ce9b3fb8"
  }, {
    "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" : "2026-10-02T18:53:05Z",
    "nvd_published_at" : null
  }
}