{"id":"CVE-2026-63430","aliases":["RUSTSEC-2026-0193","GHSA-9jh8-v38h-cvhr"],"title":"mXSS in ammonia via MathML `annotation-xml` encoding strip","summary":"mXSS in ammonia via MathML `annotation-xml` encoding strip","severity":"none","vendor":"ammonia","product":"ammonia","ecosystem":"rust","affected":["ammonia >= 4.1.0, < 4.1.3"],"patched":["ammonia 4.1.3"],"published":"2026-06-30","updated":"2026-07-17","source":"OSV","sourceUrl":"https://osv.dev/vulnerability/RUSTSEC-2026-0193","references":[{"url":"https://crates.io/crates/ammonia"},{"url":"https://rustsec.org/advisories/RUSTSEC-2026-0193.html"}],"tags":["osv","rust"],"ingestedAt":"2026-07-17T19:00:52.026Z","slug":"CVE-2026-63430","body":"## Overview\n\nIf a certain set of MathML tags are enabled, an attacker can inject arbitrary JavaScript code into the user's browser.\n\nThe `annotation-xml` tag has slightly different behavior than the other \"integration point\"\ntags in MathML and SVG, but ammonia didn't handle it, so it didn't correctly\nstrip the namespace-incompatible tags.\n\nThis vulnerability only has an effect when the `math` and `annotation-xml` tags\nare both enabled, but the `encoding` attribute is disabled, because it relies\non the following sequence of steps:\n\n1. User writes code like `<math><annotation-xml encoding=\"text/html\"><gadget></annotation-xml></math>`.\n2. Namespace filtering checks the DOM, and it passes. `<gadget>` is parsed as HTML.\n3. 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.\n4. The gadget is written in such a way that it exploits the parsing differences between HTML and MathML.\n\nAdditionally, the gadget can only be written using a tag that is parsed as raw text in HTML.\nThese [elements] are:\n\n* title\n* textarea\n* xmp\n* iframe\n* noembed\n* noframes\n* plaintext\n* noscript\n* style\n* script\n\nApplications that do not explicitly allow any of these tags should not be affected, since none are allowed by default.\n\n[elements]: https://github.com/servo/html5ever/blob/045a0378f2b0f8d4a350793899cf722a2a9b3d11/html5ever/src/tree_builder/rules.rs\n\n---\n\n**Discovered by:** [Ivan Ivančić](https://ivan09999.github.io/ammonia-mxss) · **Date:** 2026-06-29 · Found via local differential analysis and source review of ammonia's sanitisation pipeline; no third-party systems were tested.\n\n## Affected packages\n\n- `ammonia >= 4.1.0, < 4.1.3`\n\n## Remediation\n\nUpgrade to a patched release:\n\n- `ammonia 4.1.3`","depth":"sunlit","depthScore":3,"depthScoreParts":{"impact":2.8,"likelihood":0,"exploitation":0,"ransomware":0},"changes":[]}