{"id":"CVE-2026-41239","aliases":["GHSA-crv5-9vww-q3g8"],"url":"https://o3.security/vulnerability/CVE-2026-41239","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, ' '); // {{...}} -> ' '\n content = content.replace(ERB_EXPR, ' '); // <%...%> -> ' '\n content = content.replace(TMPLIT_EXPR, ' '); // ${... -> ' '\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; // <-- 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 `{{constructor.constructor('alert(1)')()}}` exploits this:\n\n1. Parser creates: `TEXT(\"{\")` → `` → `TEXT(\"{payload}\")` → `` → `TEXT(\"}\")` — no single node contains `{{`, so pass #1 misses it\n2. `` 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\n\n\n\n \n \n \n\n\n\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) => {\n+ html = stringReplace(html, expr, ' ');\n+ });\n+ body.innerHTML = html;\n+ }\n+\n if (RETURN_DOM_FRAGMENT) {\n returnNode = createDocumentFragment.call(body.ownerDocument);\n```","published":"2026-04-23T14:47:56.129Z","modified":"2026-08-12T03:51:12.667935216Z","cvss":{"score":6.8,"severity":"MEDIUM","vector":"CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:H/I:H/A:N"},"epss":{"score":0.00248,"percentile":0.16262,"asOf":"2026-08-12"},"cisaKev":null,"exploitsKnown":0,"affectedPackages":[{"ecosystem":"npm","name":"dompurify","fixedVersion":"3.4.0"}],"fix":null,"references":[{"type":"WEB","url":"https://github.com/cure53/DOMPurify/releases/tag/3.4.0"},{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/41xxx/CVE-2026-41239.json"},{"type":"ADVISORY","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"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-08-12T03:51:12.667935216Z"}}