RUSTSEC-2026-0193: mXSS in ammonia via MathML `annotation-xml` encoding strip

What to do

1. See which version you have: cargo tree -i ammonia (it also shows which of your dependencies pulls it in).
2. Update within your version requirement: cargo update -p ammonia. If the fix is a newer major or minor than your requirement allows, raise it in Cargo.toml (for example ammonia = "4.1.3") or update the dependency that pulls it in.
3. Check again with cargo audit, or paste your Cargo.lock into our Cargo.toml checker.

Details from the advisory

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.

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**Discovered by:** Ivan Ivančić · **Date:** 2026-06-29 · Found via local differential analysis and source review of ammonia's sanitisation pipeline; no third-party systems were tested.

References

• Package: crates.io/crates/ammonia
• Advisory: rustsec.org/advisories/RUSTSEC-2026-0193.html

From the RustSec Advisory Database (CC0-1.0) via the OSV export, downloaded Oct 24, 2026. Severity score computed from the published CVSS vector.

Other advisories for ammonia

• Vulnerability HighRUSTSEC-2019-0001 · 2019-04-27Uncontrolled recursion leads to abort in HTML serialization
• VulnerabilityRUSTSEC-2021-0074 · 2021-07-08Incorrect handling of embedded SVG and MathML leads to mutation XSS
• VulnerabilityRUSTSEC-2021-0157 · 2021-10-10Incorrect escaping for `url("")` functions in CSS style attributes
• VulnerabilityRUSTSEC-2021-0158 · 2021-10-10XSS in ammonia via SVG `animateMotion`, `animateTransform`, and/or `animateColor` tags
• VulnerabilityRUSTSEC-2022-0003 · 2022-01-19Space bug in `clean_text`
• VulnerabilityRUSTSEC-2025-0071 · 2025-09-21Incorrect handling of embedded SVG and MathML leads to mutation XSS after removal
• VulnerabilityRUSTSEC-2026-0213 · 2026-07-21XSS in ammonia via SVG `animate` and `set` animation tags
• VulnerabilityRUSTSEC-2026-0373 · 2026-10-16Missing case-insensitive matching for CSS url function

Full security history of ammonia