GHSA-cg2g-xh64-v4q7 DOMPurify SAFE_FOR_TEMPLATES bypass: split template expressions survive in attached shadow root content on DOM output modes

• npm dompurify: >= 3.4.4, < 3.4.17 → fixed in 3.4.17

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. ```text _scrubTemplateExpressions (src/purify.ts:1717-1751): • node.normalize() at line 1718 does not merge text nodes inside shadow roots (DOM spec). • NodeIterator at lines 1723-1731 does not descend into shadow trees (DOM spec). • Explicit recursion at lines 1743-1750 enters .content only, via querySelectorAll('template'); there is no equivalent recursion into attached shadow roots. Shadow roots are sanitized earlier by _sanitizeShadowDOM (src/purify.ts:2710), which applies the per-node strip but never performs the post-walk normalize-and-rescrub. ``` Shadow root text nodes are therefore sanitized only per-node and never re-examined as a merged unit. Impact An 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 `` 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. Proof of Concept ```text Preconditions: • DOMPurify built from the affected source (dist/purify.js or an ESM/CJS bundle). • A browser (or DOM implementation) with shadow DOM support. • Application constructs a DOM node with an attached shadow root before sanitizing, and later evaluates the shadow root's text content as a template. ``` Build 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`: ```javascript const host = document.createElement('div'); const root = host.attachShadow({ mode: 'open' }); // Split "${alert(document.domain)}" so no single fragment matches a regex. // is not in the default ALLOWED_TAGS and is removed during sanitization, // leaving the surrounding text-node siblings adjacent. root.appendChild(document.createTextNode('$')); root.appendChild(document.createElement('object')); root.appendChild(document.createTextNode('{alert(document.domain)}')); DOMPurify.sanitize(host, { IN_PLACE: true, SAFE_FOR_TEMPLATES: true }); // After the disallowed element is removed, the fragments are adjacent and survive. host.shadowRoot.normalize(); console.log(host.shadowRoot.textContent); ``` ```text ${alert(document.domain)} ``` A downstream template evaluator reading `shadowRoot.textContent` (for example `new Function('return `' + content + '`')()` or any `${...}`/`{{...}}` engine) then evaluates the fully formed expression. Attack Chain 1. 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. 2. Control: The attacker controls the shadow root's text nodes and the disallowed separator elements between them; each fragment (`$`, `{alert(document.domain)}`) is a valid, individually benign text node. 3. Path: `_sanitizeAttachedShadowRoots` (`src/purify.ts:2977`) feeds the shadow root into `_sanitizeShadowDOM` (`src/purify.ts:2710`), which strips expressions per text node (`src/purify.ts:2246`) and removes the disallowed separator elements, leaving the fragments adjacent. 4. Guard: The post-walk safety net `_scrubTemplateExpressions` (`src/purify.ts:3131-3132`) is the only pass that would catch the merged expression, but it never recurses into shadow roots (`src/purify.ts:1717-1751` recurses into `.content` only). 5. Primitive: The adjacent fragments serialize as a single concatenated string on `shadowRoot.textContent` / `shadowRoot.innerHTML`, reconstituting the complete expression. 6. Result: A downstream template engine evaluates the attacker-chosen expression, achieving script execution (or template injection) in the engine's context. Bypass Evidence The relevant protection is `SAFE_FOR_TEMPLATES`, specifically the post-walk pass that defends against expressions that only form after adjacent text nodes merge. That defense was previously extended to `.content` under CVE-2026-65900 (GHSA-gvmj-g25r-r7wr) by adding `querySelectorAll('template')` recursion at `src/purify.ts:1743-1750`. The strongest challenge is whether any code path performs the equivalent merge-and-rescrub inside shadow roots. Enumerating every call site of `_scrubTemplateExpressions` (`src/purify.ts:1717`, `1747`, `3132`, `3141`) shows none receive a shadow root, and `_sanitizeShadowDOM` (`src/purify.ts:2710`) never calls `_scrubTemplateExpressions` or `normalize()`. The per-node strip cannot compensate: `MUSTACHE_EXPR` (`src/regexp.ts:3`) requires `{{` or `}}`, `TMPLIT_EXPR` (`src/regexp.ts:5`) requires `${`, and `ERB_EXPR` (`src/regexp.ts:4`) requires ``, none of which appear in a single-character fragment. The CVE-2026-65900 regression tests exercise `.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. Affected Versions • Ecosystem: `npm` • Package: `dompurify` • Confirmed affected range: `>= 3.4.4, <= 3.4.16` • Latest release checked: `3.4.16` (npm registry) • Fix status: not fixed The 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…