{"id":"GHSA-cg2g-xh64-v4q7","summary":"DOMPurify SAFE_FOR_TEMPLATES bypass: split template expressions survive in attached shadow root content on DOM output modes","details":"## Summary\n\nWhen 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 `\u003c%...%\u003e` 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 `\u003ctemplate\u003e.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.\n\n## Root Cause\n\n`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.\n\n```text\n_scrubTemplateExpressions (src/purify.ts:1717-1751):\n  - node.normalize() at line 1718 does not merge text nodes inside shadow roots (DOM spec).\n  - NodeIterator at lines 1723-1731 does not descend into shadow trees (DOM spec).\n  - Explicit recursion at lines 1743-1750 enters \u003ctemplate\u003e.content only, via\n    querySelectorAll('template'); there is no equivalent recursion into attached\n    shadow roots.\nShadow roots are sanitized earlier by _sanitizeShadowDOM (src/purify.ts:2710), which\napplies the per-node strip but never performs the post-walk normalize-and-rescrub.\n```\n\nShadow root text nodes are therefore sanitized only per-node and never re-examined as a merged unit.\n\n## Impact\n\nAn attacker who controls DOM-node input containing an attached shadow root can inject template expressions that survive sanitization and are later evaluated by the application's template engine. In client-side contexts this yields cross-site scripting; in a server-side template context it can escalate to template injection. Exploitation requires non-default application configuration: the application must enable `SAFE_FOR_TEMPLATES` and a DOM-returning mode, pass a DOM node that already has an attached shadow root, and feed the sanitized shadow root content into a template evaluator. The shadow root does not have to be built programmatically with `attachShadow`: in engines that support declarative shadow DOM, attacker-supplied markup containing `\u003ctemplate shadowrootmode=\"open\"\u003e` parsed through `setHTMLUnsafe` or `Document.parseHTMLUnsafe` produces the same attached shadow root, so a plain HTML string reaches the gap once the application is configured this way. Demonstrated capability is injection of a complete, attacker-chosen expression into post-sanitization shadow root content; the resulting script execution and any confidentiality or integrity consequence depend on the downstream engine's evaluation context.\n\n## Proof of Concept\n\n```text\nPreconditions:\n  - DOMPurify built from the affected source (dist/purify.js or an ESM/CJS bundle).\n  - A browser (or DOM implementation) with shadow DOM support.\n  - Application constructs a DOM node with an attached shadow root before sanitizing,\n    and later evaluates the shadow root's text content as a template.\n```\n\nBuild a host element whose shadow root holds a template expression split across text nodes separated by disallowed elements, then sanitize in place with `SAFE_FOR_TEMPLATES`:\n\n```javascript\nconst host = document.createElement('div');\nconst root = host.attachShadow({ mode: 'open' });\n\n// Split \"${alert(document.domain)}\" so no single fragment matches a regex.\n// \u003cobject\u003e is not in the default ALLOWED_TAGS and is removed during sanitization,\n// leaving the surrounding text-node siblings adjacent.\nroot.appendChild(document.createTextNode('$'));\nroot.appendChild(document.createElement('object'));\nroot.appendChild(document.createTextNode('{alert(document.domain)}'));\n\nDOMPurify.sanitize(host, { IN_PLACE: true, SAFE_FOR_TEMPLATES: true });\n\n// After the disallowed element is removed, the fragments are adjacent and survive.\nhost.shadowRoot.normalize();\nconsole.log(host.shadowRoot.textContent);\n```\n\n```text\n${alert(document.domain)}\n```\n\nA downstream template evaluator reading `shadowRoot.textContent` (for example `new Function('return `' + content + '`')()` or any `${...}`/`{{...}}` engine) then evaluates the fully formed expression.\n\n## Attack Chain\n\n1. **Exposure:** The application calls `DOMPurify.sanitize(node, cfg)` (`src/purify.ts:2854`) with `SAFE_FOR_TEMPLATES: true` and `IN_PLACE: true` (or a shadow-aware `RETURN_DOM`), passing a DOM node that already carries an attached shadow root.\n2. **Control:** The attacker controls the shadow root's text nodes and the disallowed separator elements between them; each fragment (`$`, `{alert(document.domain)}`) is a valid, individually benign text node.\n3. **Path:** `_sanitizeAttachedShadowRoots` (`src/purify.ts:2977`) feeds the shadow root into `_sanitizeShadowDOM` (`src/purify.ts:2710`), which strips expressions per text node (`src/purify.ts:2246`) and removes the disallowed separator elements, leaving the fragments adjacent.\n4. **Guard:** The post-walk safety net `_scrubTemplateExpressions` (`src/purify.ts:3131-3132`) is the only pass that would catch the merged expression, but it never recurses into shadow roots (`src/purify.ts:1717-1751` recurses into `\u003ctemplate\u003e.content` only).\n5. **Primitive:** The adjacent fragments serialize as a single concatenated string on `shadowRoot.textContent` / `shadowRoot.innerHTML`, reconstituting the complete expression.\n6. **Result:** A downstream template engine evaluates the attacker-chosen expression, achieving script execution (or template injection) in the engine's context.\n\n## Bypass Evidence\n\nThe relevant protection is `SAFE_FOR_TEMPLATES`, specifically the post-walk pass that defends against expressions that only form after adjacent text nodes merge. That defense was previously extended to `\u003ctemplate\u003e.content` under CVE-2026-65900 (GHSA-gvmj-g25r-r7wr) by adding `querySelectorAll('template')` recursion at `src/purify.ts:1743-1750`. The strongest challenge is whether any code path performs the equivalent merge-and-rescrub inside shadow roots. Enumerating every call site of `_scrubTemplateExpressions` (`src/purify.ts:1717`, `1747`, `3132`, `3141`) shows none receive a shadow root, and `_sanitizeShadowDOM` (`src/purify.ts:2710`) never calls `_scrubTemplateExpressions` or `normalize()`. The per-node strip cannot compensate: `MUSTACHE_EXPR` (`src/regexp.ts:3`) requires `{{` or `}}`, `TMPLIT_EXPR` (`src/regexp.ts:5`) requires `${`, and `ERB_EXPR` (`src/regexp.ts:4`) requires `\u003c%` or `%\u003e`, none of which appear in a single-character fragment. The CVE-2026-65900 regression tests exercise `\u003ctemplate\u003e.content` only; no test covers shadow root content under `SAFE_FOR_TEMPLATES`. The gap is therefore an unfixed case on a distinct remediation boundary (shadow roots) rather than a bypass of the delivered template-content fix.\n\n## Affected Versions\n\n- **Ecosystem:** `npm`\n- **Package:** `dompurify`\n- **Confirmed affected range:** `\u003e= 3.4.4, \u003c= 3.4.16`\n- **Latest release checked:** `3.4.16` ([npm registry](https://www.npmjs.com/package/dompurify/v/3.4.16))\n- **Fix status:** not fixed\n\nThe lower bound 3.4.4 is the release that introduced both the attached-shadow-root sanitization pass and the `_scrubTemplateExpressions` safety net; releases 3.4.4 through 3.4.7 carry the safety net with no content-container recursion, and 3.4.8 onward add `\u003ctemplate\u003e.content` recursion but still omit shadow root recursion. No published release and no current development revision contains a shadow-root fix.\n\n## Suggested Fix\n\nEnforce the same post-walk normalize-and-rescrub invariant on attached shadow roots that already applies to `\u003ctemplate\u003e.content`. After `_sanitizeShadowDOM` finishes walking a shadow root, run `_scrubTemplateExpressions` over that shadow root so adjacent text nodes created by element removal are merged and re-stripped; alternatively, have `_scrubTemplateExpressions` discover and recurse into attached shadow roots as it does for template content. The fix must cover nested shadow roots and shadow roots inside `\u003ctemplate\u003e.content`, and it must handle all three expression dialects, since any disallowed element can serve as the separator that produces adjacent fragments.\n\n---\n\nReported by zx (GitHub: @manus-pi).","modified":"2026-10-22T16:45:03.731164651Z","published":"2026-10-22T16:25:31Z","database_specific":{"github_reviewed_at":"2026-10-22T16:25:31Z","nvd_published_at":null,"cwe_ids":["CWE-79"],"severity":"CRITICAL","github_reviewed":true},"references":[{"type":"WEB","url":"https://github.com/cure53/DOMPurify/security/advisories/GHSA-cg2g-xh64-v4q7"},{"type":"WEB","url":"https://github.com/cure53/DOMPurify/pull/1672"},{"type":"WEB","url":"https://github.com/cure53/DOMPurify/commit/a77a3e78b59f72fa4d1c83f22e121ad9cc7d3575"},{"type":"PACKAGE","url":"https://github.com/cure53/DOMPurify"},{"type":"WEB","url":"https://github.com/cure53/DOMPurify/releases/tag/3.4.17"}],"affected":[{"package":{"name":"dompurify","ecosystem":"npm","purl":"pkg:npm/dompurify"},"ranges":[{"type":"SEMVER","events":[{"introduced":"3.4.4"},{"fixed":"3.4.17"}]}],"database_specific":{"last_known_affected_version_range":"\u003c= 3.4.16","source":"https://github.com/github/advisory-database/blob/main/advisories/github-reviewed/2026/10/GHSA-cg2g-xh64-v4q7/GHSA-cg2g-xh64-v4q7.json"}}],"schema_version":"1.9.0","severity":[{"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"}]}