6.1

CVSS Score

3.1

-

CVSS Score

Basic Information

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CVE-2026-49458

CVE-2026-49458: DOMPurify: Cross-realm IN_PLACE sanitization leaves executable markup intact via realm-bound `instanceof` checks

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.

(GitHub Advisory)

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CVE-2026-49458

CVE-2026-49458:

6.1

CVSS Score

3.1

-

CVSS Score

Basic Information

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Technical Details

SSVC / BOD 26-04 ↗

Exploitation pocAutomatable NoTech Impact Partial

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Package Name	Ecosystem	Vulnerable Versions	First Patched Version
dompurify	npm	<= 3.4.5	3.4.6

Technical Details

Vulnerability Intelligence
Miggo AI

Root Cause Analysis

The vulnerability lies in DOMPurify's handling of DOM nodes from different JavaScript realms when using the IN_PLACE: true option. The library correctly accepts nodes from foreign realms (like iframes) for sanitization. However, several internal functions responsible for security checks use the instanceof operator to determine the type of a node (e.g., instanceof HTMLFormElement, instanceof DocumentFragment).

The instanceof operator is realm-bound, meaning it checks the object's prototype chain against the constructors of the current realm. When a node from a foreign realm is passed, these instanceof checks return false because the node was constructed in a different realm with its own set of constructors.

This leads to a failure in protection mechanisms. Specifically:

_isClobbered fails to detect clobbered <form> elements, allowing attribute sanitization to be bypassed.
_sanitizeShadowDOM fails to recognize the content of foreign <template> elements as DocumentFragments, skipping their sanitization.
_sanitizeAttachedShadowRoots fails to identify foreign shadow roots as DocumentFragments, skipping the sanitization of the entire shadow DOM.

As a result, attacker-controlled markup within these structures remains intact and can lead to Cross-Site Scripting (XSS) when the sanitized node is used in the application. The patch replaces these realm-bound instanceof checks with realm-agnostic methods, such as checking the nodeType or nodeName properties, to ensure consistent behavior across realms.

Vulnerable functions

_isClobbered

src/purify.ts

The function uses `element instanceof HTMLFormElement` and `element.attributes instanceof NamedNodeMap` to check for DOM clobbering. These checks are realm-bound and will fail for nodes created in a different realm (e.g., an iframe), causing the function to incorrectly return `false` (not clobbered). This allows attribute sanitization to be bypassed, leading to XSS.

_sanitizeShadowDOM

src/purify.ts

The function uses `instanceof DocumentFragment` to check if the content of a `<template>` element should be sanitized. This check is realm-bound and fails for templates created in a different realm. As a result, the content of the template is not sanitized, allowing malicious code to be executed when the template is used.

_sanitizeAttachedShadowRoots

src/purify.ts

This function uses `instanceof DocumentFragment` to check if an element's attached shadow root needs sanitization. This check is realm-bound and fails for shadow roots created in a different realm. This causes the entire shadow DOM to be skipped during sanitization, leaving any malicious content within it intact.

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WAF Rule

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Reasoning

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