CVE-2026-41238: DOMPurify: Prototype Pollution to XSS Bypass via CUSTOM_ELEMENT_HANDLING Fallback

Summary

DOMPurify versions 3.0.1 through 3.3.3 (latest) are vulnerable to a prototype pollution-based XSS bypass. When an application uses DOMPurify.sanitize() with the default configuration (no CUSTOMELEMENTHANDLING option), a prior prototype pollution gadget can inject permissive tagNameCheck and attributeNameCheck regex values into Object.prototype, causing DOMPurify to allow arbitrary custom elements with arbitrary attributes — including event handlers — through sanitization.

Affected Versions

- 3.0.1 through 3.3.3 (current latest) — all affected - 3.0.0 and all 2.x versions — NOT affected (used Object.create(null) for initialization, no || {} reassignment) - The vulnerable || {} reassignment was introduced in the 3.0.0→3.0.1 refactor - This is distinct from GHSA-cj63-jhhr-wcxv (USEPROFILES Array.prototype pollution, fixed in 3.3.2) - This is distinct from CVE-2024-45801 / GHSA-mmhx-hmjr-r674 (depth prototype pollution, fixed in 3.1.3)

Root Cause

In purify.js at line 590, during config parsing:

javascript CUSTOMELEMENTHANDLING = cfg.CUSTOMELEMENTHANDLING || {};

When no CUSTOMELEMENTHANDLING is specified in the config (the default usage pattern), cfg.CUSTOMELEMENTHANDLING is undefined, and the fallback {} is used. This plain object inherits from Object.prototype.

Lines 591-598 then check cfg.CUSTOMELEMENTHANDLING (the original config property) — which is undefined — so the conditional blocks that would set tagNameCheck and attributeNameCheck from the config are never entered.

As a result, CUSTOMELEMENTHANDLING.tagNameCheck and CUSTOMELEMENTHANDLING.attributeNameCheck resolve via the prototype chain. If an attacker has polluted Object.prototype.tagNameCheck and Object.prototype.attributeNameCheck with permissive values (e.g., /./), these polluted values flow into DOMPurify's custom element validation at lines 973-977 and attribute validation, causing all custom elements and all attributes to be allowed.

Impact

- Attack type: XSS bypass via prototype pollution chain - Prerequisites: Attacker must have a prototype pollution primitive in the same execution context (e.g., vulnerable version of lodash, jQuery.extend, query-string parser, deep merge utility, or any other PP gadget) - Config required: Default. No special DOMPurify configuration needed. The standard DOMPurify.sanitize(userInput) call is affected. - Payload: Any HTML custom element (name containing a hyphen) with event handler attributes survives sanitization

Proof of Concept

javascript // Step 1: Attacker exploits a prototype pollution gadget elsewhere in the application Object.prototype.tagNameCheck = /./; Object.prototype.attributeNameCheck = /./;

// Step 2: Application sanitizes user input with DEFAULT config const clean = DOMPurify.sanitize('<x-x onfocus=alert(document.cookie) tabindex=0 autofocus>');

// Step 3: "Sanitized" output still contains the event handler console.log(clean); // Output: <x-x onfocus="alert(document.cookie)" tabindex="0" autofocus="">

// Step 4: When injected into DOM, XSS executes document.body.innerHTML = clean; // alert() fires

Tested configurations that are vulnerable:

| Call Pattern | Vulnerable? | |---|---| | DOMPurify.sanitize(input) | YES | | DOMPurify.sanitize(input, {}) | YES | | DOMPurify.sanitize(input, { CUSTOMELEMENTHANDLING: null }) | YES | | DOMPurify.sanitize(input, { CUSTOMELEMENTHANDLING: {} }) | NO (explicit object triggers L591 path) |

Suggested Fix

Change line 590 from: javascript CUSTOMELEMENTHANDLING = cfg.CUSTOMELEMENTHANDLING || {};

To: javascript CUSTOMELEMENTHANDLING = cfg.CUSTOMELEMENTHANDLING || create(null);

The create(null) function (already used elsewhere in DOMPurify, e.g., in clone()) creates an object with no prototype, preventing prototype chain inheritance.

Alternative application-level mitigation:

Applications can protect themselves by always providing an explicit CUSTOMELEMENTHANDLING in their config:

javascript DOMPurify.sanitize(input, { CUSTOMELEMENTHANDLING: { tagNameCheck: null, attributeNameCheck: null } });

Timeline

- 2026-04-04: Vulnerability discovered during automated DOMPurify fuzzing research (Fermat project) - 2026-04-04: Confirmed in Chrome browser with DOMPurify 3.3.3 - 2026-04-04: Verified distinct from GHSA-cj63-jhhr-wcxv and CVE-2024-45801 - 2026-04-04: Advisory drafted, responsible disclosure initiated

Credit

https://github.com/trace37labs