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. [elements]: https://github.com/servo/html5ever/blob/045a0378f2b0f8d4a350793899cf722a2a9b3d11/html5ever/src/tree_builder/rules.rs ### 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. ### Patches - https://github.com/rust-ammonia/ammonia/pull/245 (released in 4.1.3) - https://github.com/rust-ammonia/ammonia/pull/244 (released in 4.0.2) - https://github.com/rust-ammonia/ammonia/pull/243 (released in 3.3.2) ### Workarounds Call <code>builder.[add_clean_content_tags](https://docs.rs/ammonia/latest/ammonia/struct.Builder.html#method.add_clean_content_tags)("annotation-xml")</code> to remove the annotation-xml element and all of its children, including the XSS gadget.

Source published

22 Oct 2026 14:06:10 UTC

Source modified

22 Oct 2026 14:16:23 UTC

Observed here

24 Oct 2026 13:19:54 UTC

Source terms

GitHub Advisory Database · CC-BY-4.0

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CVSS_V4 · CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:N/SC:L/SI:L/SA:N

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