CVE-2026-63430: 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.