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 CUSTOM_ELEMENT_HANDLING 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 (USE_PROFILES 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:

CUSTOM_ELEMENT_HANDLING = cfg.CUSTOM_ELEMENT_HANDLING || {};

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

Lines 591-598 then check cfg.CUSTOM_ELEMENT_HANDLING (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, CUSTOM_ELEMENT_HANDLING.tagNameCheck and CUSTOM_ELEMENT_HANDLING.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

// 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, { CUSTOM_ELEMENT_HANDLING: null }) | YES | | DOMPurify.sanitize(input, { CUSTOM_ELEMENT_HANDLING: {} }) | NO (explicit object triggers L591 path) |

Suggested Fix

Change line 590 from:

CUSTOM_ELEMENT_HANDLING = cfg.CUSTOM_ELEMENT_HANDLING || {};

To:

CUSTOM_ELEMENT_HANDLING = cfg.CUSTOM_ELEMENT_HANDLING || 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 CUSTOM_ELEMENT_HANDLING in their config:

DOMPurify.sanitize(input, {
  CUSTOM_ELEMENT_HANDLING: {
    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