CVE-2026-63430

CVE-2026-63430 is a medium-severity cross-site scripting (XSS) vulnerability in ammonia (rust), affecting versions < 3.3.2. It is fixed in 3.3.2, 4.0.2, 4.1.3.

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Summary

ammonia: mXSS via MathML annotation-xml encoding strip

If a certain set of MathML tags are enabled, an attacker can inject arbitrary JavaScript code into the user's browser.

The annotation-xml tag has slightly different behavior than the other "integration point"
tags in MathML and SVG, but ammonia didn't handle it, so it didn't correctly
strip the namespace-incompatible tags.

This vulnerability only has an effect when the math and annotation-xml tags
are both enabled, but the encoding attribute is disabled, because it relies
on the following sequence of steps:

  1. User writes code like <math><annotation-xml encoding="text/html"><gadget></annotation-xml></math>.
  2. Namespace filtering checks the DOM, and it passes. <gadget> is parsed as HTML.
  3. Attribute filter strips it down to <math><annotation-xml><gadget></annotation-xml></math>. Because the encoding attribute is gone, <gadget> is now parsed as MathML.
  4. The gadget is written in such a way that it exploits the parsing differences between HTML and MathML.

Additionally, the gadget can only be written using a tag that is parsed as raw text in HTML.
These elements are:

  • title
  • textarea
  • xmp
  • iframe
  • noembed
  • noframes
  • plaintext
  • noscript
  • style
  • script

Applications that do not explicitly allow any of these tags should not be affected, since none are allowed by default.

Workarounds

Call builder.add_clean_content_tags("annotation-xml") to remove the annotation-xml element and all of its children, including the XSS gadget.

Impact

Low: Requires both MathML and at least one raw-text HTML tag to be enabled, and most websites don't enable either of those.

Untrusted input is rendered as active markup in a victim's browser, which can run script in their session. Typical impact: session or credential theft, and actions taken as the user.

Affected versions

ammonia (< 3.3.2) ammonia (>= 4.0.0, < 4.0.2) ammonia (>= 4.1.0, < 4.1.3)

Security releases

ammonia → 3.3.2 (rust) ammonia → 4.0.2 (rust) ammonia → 4.1.3 (rust)

Kodem intelligence

Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.

Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.

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Remediation advice

Frequently Asked Questions

  1. What is CVE-2026-63430? CVE-2026-63430 is a medium-severity cross-site scripting (XSS) vulnerability in ammonia (rust), affecting versions < 3.3.2. It is fixed in 3.3.2, 4.0.2, 4.1.3. Untrusted input is rendered as active markup in a victim's browser, which can run script in their session.
  2. Which versions of ammonia are affected by CVE-2026-63430? ammonia (rust) versions < 3.3.2 is affected.
  3. Is there a fix for CVE-2026-63430? Yes. CVE-2026-63430 is fixed in 3.3.2, 4.0.2, 4.1.3. Upgrade to this version or later.
  4. Is CVE-2026-63430 exploitable, and should I be worried? Whether CVE-2026-63430 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
  5. What actually determines whether CVE-2026-63430 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
  6. How do I fix CVE-2026-63430?
    • Upgrade ammonia to 3.3.2 or later
    • Upgrade ammonia to 4.0.2 or later
    • Upgrade ammonia to 4.1.3 or later

Other vulnerabilities in ammonia

CVE-IDSeverityPackage summary
CVE-2026-102342Mediumammonia: Ammonia: Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')GHSA-MM7X-QFJJ-5G2CLowammonia: Ammonia incorrectly handles embedded SVG and MathML leading to mutation XSS after removalGHSA-P2G9-94WH-65C2Mediumammonia: Space bug in `clean_text`CVE-2021-38193Mediumammonia: Cross-site Scripting in ammoniaCVE-2019-15542Highammonia: Uncontrolled recursion in ammonia

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