Dulwich: Symlink write-through in checkout(paths=[]) via raw os.open bypasses all symlink protections

{
  "id": "50e9e6613ae2a25b4881b695e2354cf9010b1634b131f424a33ae2efc86a600f",
  "url": "https://osv.dev/vulnerability/GHSA-8w8g-wq8h-fq33",
  "method": "structured",
  "payload": {
    "kind": "advisory",
    "title": "Dulwich: Symlink write-through in checkout(paths=[]) via raw os.open bypasses all symlink protections",
    "summary": "## Summary Dulwich'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. ## Root Cause At `dulwich/porcelain/__init__.py:5661-5675`, the `checkout(paths=[...])` implementation: ```python file_path = _checked_worktree_path(r, path) os.makedirs(os.path.dirname(file_path), exist_ok=True) flags = os.O_WRONLY | os.O_CREAT | os.O_TRUNC with os.fdopen(os.open(file_path, flags, mode), \"wb\") as f: f.write(obj.data) ``` `_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. ## Impact An 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. ## Attack Scenario 1. Attacker creates a repository where HEAD has `trigger` as a symlink (mode 120000, content `../../.git/hooks/post-checkout`), and tag `",
    "evidence": "osv:GHSA-8w8g-wq8h-fq33",
    "products": [
      {
        "name": {
          "value": "dulwich",
          "evidence": "/affected/0/package/name"
        },
        "prerequisites": [],
        "fixed_versions": [
          {
            "value": "1.2.8",
            "evidence": "/affected/0/ranges"
          }
        ],
        "affected_versions": [
          {
            "value": "0.24.0",
            "evidence": "/affected/0/versions"
          },
          {
            "value": "0.24.1",
            "evidence": "/affected/0/versions"
          },
          {
            "value": "0.24.10",
            "evidence": "/affected/0/versions"
          },
          {
            "value": "0.24.2",
            "evidence": "/affected/0/versions"
          },
          {
            "value": "0.24.3",
            "evidence": "/affected/0/versions"
          },
          {
            "value": "0.24.4",
            "evidence": "/affected/0/versions"
          },
          {
            "value": "0.24.5",
            "evidence": "/affected/0/versions"
          },
          {
            "value": "0.24.6",
            "evidence": "/affected/0/versions"
          },
          {
            "value": "0.24.7",
            "evidence": "/affected/0/versions"
          },
          {
            "value": "0.24.8",
            "evidence": "/affected/0/versions"
          },
          {
            "value": "0.24.9",
            "evidence": "/affected/0/versions"
          },
          {
            "value": "0.25.0",
            "evidence": "/affected/0/versions"
          },
          {
            "value": "0.25.1",
            "evidence": "/affected/0/versions"
          },
          {
            "value": "0.25.2",
            "evidence": "/affected/0/versions"
          },
          {
            "value": "1.0.0",
            "evidence": "/affected/0/versions"
          },
          {
            "value": "1.1.0",
            "evidence": "/affected/0/versions"
          },
          {
            "value": "1.2.0",
            "evidence": "/affected/0/versions"
          },
          {
            "value": "1.2.1",
            "evidence": "/affected/0/versions"
          },
          {
            "value": "1.2.2",
            "evidence": "/affected/0/versions"
          },
          {
            "value": "1.2.3",
            "evidence": "/affected/0/versions"
          },
          {
            "value": "1.2.4",
            "evidence": "/affected/0/versions"
          },
          {
            "value": "1.2.5",
            "evidence": "/affected/0/versions"
          },
          {
            "value": "1.2.6",
            "evidence": "/affected/0/versions"
          },
          {
            "value": "1.2.7",
            "evidence": "/affected/0/versions"
          }
        ]
      }
    ],
    "severity": "high",
    "structured": {
      "id": "GHSA-8w8g-wq8h-fq33",
      "aliases": [
        "CVE-2026-114096",
        "GHSA-8w8g-wq8h-fq33"
      ],
      "metrics": [
        {
          "type": "CVSS_V3",
          "score": "CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:C/C:H/I:H/A:H"
        }
      ],
      "affected": [
        {
          "ranges": [
            {
              "type": "ECOSYSTEM",
              "events": [
                {
                  "introduced": "0.24.0"
                },
                {
                  "fixed": "1.2.8"
                }
              ]
            }
          ],
          "package": {
            "name": "dulwich",
            "purl": "pkg:pypi/dulwich",
            "ecosystem": "PyPI"
          },
          "versions": [
            "0.24.0",
            "0.24.1",
            "0.24.10",
            "0.24.2",
            "0.24.3",
            "0.24.4",
            "0.24.5",
            "0.24.6",
            "0.24.7",
            "0.24.8",
            "0.24.9",
            "0.25.0",
            "0.25.1",
            "0.25.2",
            "1.0.0",
            "1.1.0",
            "1.2.0",
            "1.2.1",
            "1.2.2",
            "1.2.3",
            "1.2.4",
            "1.2.5",
            "1.2.6",
            "1.2.7"
          ],
          "database_specific": {
            "source": "https://github.com/github/advisory-database/blob/main/advisories/github-reviewed/2026/10/GHSA-8w8g-wq8h-fq33/GHSA-8w8g-wq8h-fq33.json",
            "last_known_affected_version_range": "<= 1.2.7"
          }
        }
      ],
      "modified": "2026-10-21T10:57:28.880982828Z",
      "provider": "osv",
      "references": [
        "https://github.com/jelmer/dulwich/security/advisories/GHSA-8w8g-wq8h-fq33",
        "https://github.com/jelmer/dulwich/commit/9389fcb5cb9113adfc7f207d8be86a56904db3e8",
        "https://github.com/jelmer/dulwich",
        "https://github.com/jelmer/dulwich/releases/tag/dulwich-1.2.8"
      ],
      "provider_metadata": {
        "cwe_ids": [
          "CWE-59",
          "CWE-61"
        ],
        "severity": "HIGH",
        "github_reviewed": true,
        "nvd_published_at": null,
        "github_reviewed_at": "2026-10-02T18:53:35Z"
      }
    },
    "related_cves": [
      {
        "value": "CVE-2026-114096",
        "evidence": "/aliases"
      }
    ],
    "security_relevant": true,
    "severity_evidence": "/database_specific/severity"
  },
  "source_id": "core-osv",
  "document_id": 392988,
  "published_at": "2026-10-02T18:53:35+00:00"
}