GHSA-crv5-9vww-q3g8 DOMPurify has a SAFE_FOR_TEMPLATES bypass in RETURN_DOM mode

• npm dompurify: >= 1.0.10, < 3.4.0 → fixed in 3.4.0

Summary | Field | Value | |:------|:------| | Severity | Medium | | Affected | DOMPurify `main` at `883ac15`, introduced in v1.0.10 (`7fc196db`) | `SAFE_FOR_TEMPLATES` strips `{{...}}` expressions from untrusted HTML. This works in string mode but not with `RETURN_DOM` or `RETURN_DOM_FRAGMENT`, allowing XSS via template-evaluating frameworks like Vue 2. Technical Details DOMPurify strips template expressions in two passes: 1. Per-node — each text node is checked during the tree walk (`purify.ts:1179-1191`): ```js // pass #1: runs on every text node during tree walk if (SAFE_FOR_TEMPLATES && currentNode.nodeType === NODE_TYPE.text) { content = currentNode.textContent; content = content.replace(MUSTACHE_EXPR, ' '); // {{...}} -> ' ' content = content.replace(ERB_EXPR, ' '); // -> ' ' content = content.replace(TMPLIT_EXPR, ' '); // ${... -> ' ' currentNode.textContent = content; } ``` 2. Final string scrub — after serialization, the full HTML string is scrubbed again (`purify.ts:1679-1683`). This is the safety net that catches expressions that only form after the DOM settles. The `RETURN_DOM` path returns before pass #2 ever runs (`purify.ts:1637-1661`): ```js // purify.ts (simplified) if (RETURN_DOM) { // ... build returnNode ... return returnNode; // {constructor.constructor('alert(1)')()}}` exploits this: 1. Parser creates: `TEXT("{")` → `` → `TEXT("{payload}")` → `` → `TEXT("}")` — no single node contains `{{`, so pass #1 misses it 2. `` is not allowed, so DOMPurify removes it but keeps surrounding text 3. The three text nodes are now adjacent — `.outerHTML` reads them as `{{payload}}`, which Vue 2 compiles and executes Reproduce Open the following html in any browser and `alert(1)` pops up. ```html var dirty = '{{constructor.constructor("alert(1)")()}}'; var dom = DOMPurify.sanitize(dirty, { SAFE_FOR_TEMPLATES: true, RETURN_DOM: true }); document.body.appendChild(dom.firstChild); new Vue({ el: '#app' }); ``` Impact Any application that sanitizes attacker-controlled HTML with `SAFE_FOR_TEMPLATES: true` and `RETURN_DOM: true` (or `RETURN_DOM_FRAGMENT: true`), then mounts the result into a template-evaluating framework, is vulnerable to XSS. Recommendations Fix `normalize()` merges the split text nodes, then the same regex from the string path catches the expression. Placed before the fragment logic, this fixes both `RETURN_DOM` and `RETURN_DOM_FRAGMENT`. ```diff if (RETURN_DOM) { + if (SAFE_FOR_TEMPLATES) { + body.normalize(); + let html = body.innerHTML; + arrayForEach([MUSTACHE_EXPR, ERB_EXPR, TMPLIT_EXPR], (expr: RegExp) => { + html = stringReplace(html, expr, ' '); + }); + body.innerHTML = html; + } + if (RETURN_DOM_FRAGMENT) { returnNode = createDocumentFragment.call(body.ownerDocument); ```