DOMPurify SAFE_FOR_TEMPLATES bypass: split template expressions survive in attached shadow root content on DOM output modes

{
  "id": "afb42c92ba1b12eec283d4734ec7b898309629cbe45af2c2c5b8ccff70285fe6",
  "url": "https://osv.dev/vulnerability/GHSA-cg2g-xh64-v4q7",
  "method": "structured",
  "payload": {
    "kind": "advisory",
    "title": "DOMPurify SAFE_FOR_TEMPLATES bypass: split template expressions survive in attached shadow root content on DOM output modes",
    "summary": "## Summary When DOMPurify is configured with `SAFE_FOR_TEMPLATES: true` together with a DOM-returning mode (`RETURN_DOM: true`, `RETURN_DOM_FRAGMENT: true`, or `IN_PLACE: true`), template expressions such as `{{...}}`, `${...}`, and `<%...%>` survive sanitization inside the text content of an attached shadow root. The post-walk safety net `_scrubTemplateExpressions` (`src/purify.ts:1717`) was extended to recurse into `.content`, but it never recurses into attached shadow roots. An attacker who can supply a DOM node carrying a shadow root (the `IN_PLACE` and shadow-aware `RETURN_DOM` surfaces) can split an expression across text nodes separated by disallowed elements; after those elements are removed the fragments become adjacent and serialize into a complete expression that a downstream template engine will evaluate. ## Root Cause `SAFE_FOR_TEMPLATES` neutralizes template expressions through two mechanisms: a per-text-node strip during the main walk (`_stripTemplateExpressions`, applied in `_sanitizeElements` at `src/purify.ts:2246`), and a final post-walk pass (`_scrubTemplateExpressions`, `src/purify.ts:1717`) that normalizes adjacent text nodes and re-strips expressions that only form after merging. The per-node strip cannot catch an expression that is split into individual fragments, because each fragment fails the delimiter regexes. The post-walk pass is the only mechanism that would catch the merged form, but it is structurally unable to reach shadow root content. ```te",
    "evidence": "osv:GHSA-cg2g-xh64-v4q7",
    "products": [
      {
        "name": {
          "value": "dompurify",
          "evidence": "/affected/0/package/name"
        },
        "prerequisites": [],
        "fixed_versions": [
          {
            "value": "3.4.17",
            "evidence": "/affected/0/ranges"
          }
        ],
        "affected_versions": []
      }
    ],
    "severity": "critical",
    "structured": {
      "id": "GHSA-cg2g-xh64-v4q7",
      "aliases": [
        "CVE-2026-116804",
        "GHSA-cg2g-xh64-v4q7"
      ],
      "metrics": [
        {
          "type": "CVSS_V4",
          "score": "CVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:N/VC:H/VI:H/VA:L/SC:N/SI:N/SA:N"
        }
      ],
      "affected": [
        {
          "ranges": [
            {
              "type": "SEMVER",
              "events": [
                {
                  "introduced": "3.4.4"
                },
                {
                  "fixed": "3.4.17"
                }
              ]
            }
          ],
          "package": {
            "name": "dompurify",
            "purl": "pkg:npm/dompurify",
            "ecosystem": "npm"
          },
          "database_specific": {
            "source": "https://github.com/github/advisory-database/blob/main/advisories/github-reviewed/2026/10/GHSA-cg2g-xh64-v4q7/GHSA-cg2g-xh64-v4q7.json",
            "last_known_affected_version_range": "<= 3.4.16"
          }
        }
      ],
      "modified": "2026-10-27T10:56:08.031469803Z",
      "provider": "osv",
      "references": [
        "https://github.com/cure53/DOMPurify/security/advisories/GHSA-cg2g-xh64-v4q7",
        "https://github.com/cure53/DOMPurify/pull/1672",
        "https://github.com/cure53/DOMPurify/commit/a77a3e78b59f72fa4d1c83f22e121ad9cc7d3575",
        "https://github.com/cure53/DOMPurify",
        "https://github.com/cure53/DOMPurify/releases/tag/3.4.17"
      ],
      "provider_metadata": {
        "cwe_ids": [
          "CWE-79"
        ],
        "severity": "CRITICAL",
        "github_reviewed": true,
        "nvd_published_at": null,
        "github_reviewed_at": "2026-10-22T16:25:31Z"
      }
    },
    "related_cves": [
      {
        "value": "CVE-2026-116804",
        "evidence": "/aliases"
      }
    ],
    "security_relevant": true,
    "severity_evidence": "/database_specific/severity"
  },
  "source_id": "core-osv",
  "document_id": 461503,
  "published_at": "2026-10-22T16:25:31+00:00"
}