CVE-2026-63430

Received Received - Intake

Mutation XSS in Ammonia HTML Sanitizer

Publication date: 2026-10-15

Last updated on: 2026-10-15

Assigner: GitHub, Inc.

Description

ammonia is a Rust library for repairing and securing untrusted HTML. Prior to 3.3.2, 4.0.2, and 4.1.3, applications that explicitly allow the math tag, annotation-xml, and at least one raw-text HTML element while disallowing the encoding attribute can permit mutation cross-site scripting. ammonia does not correctly implement annotation-xml's encoding-dependent MathML integration-point behavior after attribute filtering removes encoding, so the sanitizer validates a descendant under one namespace interpretation while the browser reparses the output under another. An attacker can use that namespace transition and an allowed raw-text HTML element to execute JavaScript when a user renders the sanitized output. The required tags are not enabled by default, so applications that do not explicitly allow them are not affected. This issue is fixed in versions 3.3.2, 4.0.2, and 4.1.3.

CVSS Scores

EPSS Scores

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Meta Information

Published

2026-10-15

Last Modified

2026-10-15

Generated

2026-10-16

AI Q&A

2026-10-16

EPSS Evaluated

N/A

NVD

CVE-2026-63430

EUVD

EUVD-2026-102241

Affected Vendors & Products

Vendor	Product	Version / Range
rust-ammonia	ammonia	< 3.3.2
rust-ammonia	ammonia	>= 4.0.0, < 4.0.2
rust-ammonia	ammonia	>= 4.1.0, < 4.1.3

Helpful Resources

Exploitability

CWE

KEV

CWE ID	Description
CWE-79	The product does not neutralize or incorrectly neutralizes user-controllable input before it is placed in output that is used as a web page that is served to other users.

Attack-Flow Graph

Executive Summary

This vulnerability in the ammonia Rust library allows mutation cross-site scripting (mXSS) when specific HTML tags are enabled. The issue occurs when the math tag, annotation-xml, and at least one raw-text HTML element are allowed while the encoding attribute is disallowed. The library fails to correctly handle namespace transitions after attribute filtering, leading browsers to reparse content under a different namespace interpretation. This can allow attackers to execute JavaScript when users render sanitized output.

Impact Analysis

If you use an affected version of ammonia (before 3.3.2, 4.0.2, or 4.1.3) and explicitly allow the math tag, annotation-xml, and a raw-text HTML element while disallowing the encoding attribute, attackers could inject malicious JavaScript into your application. This could lead to unauthorized actions, data theft, or session hijacking for users who view the sanitized output.

Compliance Impact

This vulnerability could lead to data breaches or unauthorized access, violating GDPR's integrity and confidentiality principles or HIPAA's security requirements for protected health information. Organizations using affected ammonia versions may face compliance violations if the vulnerability is exploited to access sensitive data.

Mitigation Strategies

Update ammonia to version 3.3.2, 4.0.2, or 4.1.3 or later to address the vulnerability. If using a package manager, run commands like 'cargo update -p ammonia' or update via your dependency manager. Review application configurations to ensure the math tag, annotation-xml, and raw-text HTML elements are not explicitly allowed without proper encoding attributes.

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