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

• Severity: medium (CVSS 6.3 v4.0)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.6% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-10-15 · updated 2026-10-16
• Weakness: CWE-79

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.

Affected products

• Crates.Io Ammonia: from 0.0.0-0 before 3.3.2; from 4.0.0 before 4.0.2; from 4.1.0 before 4.1.3 — fixed in 3.3.2, 4.0.2, 4.1.3
• rust-ammonia ammonia: before 3.3.2; from 4.0.0, < 4.0.2; from 4.1.0, < 4.1.3

How to fix

Upgrade to a fixed version: Crates.Io Ammonia 3.3.2, 4.0.2, 4.1.3. Check the vendor advisory for mitigations.

References

• 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/pull/243
• https://github.com/rust-ammonia/ammonia/pull/244
• https://github.com/rust-ammonia/ammonia/pull/245
• 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://github.com/rust-ammonia/ammonia/security/advisories/GHSA-9jh8-v38h-cvhr
• https://ivan09999.github.io/ammonia-mxss
• https://rustsec.org/advisories/RUSTSEC-2026-0193.html

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