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Medium, CVSS 6.3amended

ammonia: mXSS via MathML annotation-xml encoding strip

CVE-2026-63430GHSA-9jh8-v38h-cvhr

What to do

Vendor fix available. Upgrade each affected package to the fixed version on its release line (ammonia 4.1.3, 4.0.2, 3.3.2), redeploy, then verify what is actually installed.

This page has no inventory attached, so it cannot say whether you are affected. A vendor fix is not proof that your deployment is fixed.

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Affected packages

  • crates.io ammonia

    Affected
    < 3.3.2>= 4.0.0, < 4.0.2>= 4.1.0, < 4.1.3
    Fixed in
    4.1.3, 4.0.2, 3.3.2

Sources

Severity and exploitation
Exploitation
No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.
CVSS 4.0
6.3 · 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 · score source
EPSS
  • CVE-2026-63430: 0.56% estimated exploitation activity in the next 30 days; percentile 46.34 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.
Source description

As published by the source; long descriptions are shortened.

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…

Dates and revisions
Published by the source
Last modified by the source
Source last checked
  1. Revision 4 · amended ·

    Amended from ghsa record GHSA-9jh8-v38h-cvhr

  2. Revision 3 · amended ·

    Amended from ghsa record GHSA-9jh8-v38h-cvhr

  3. Revision 2 · amended ·

    Reviewed OSV alias/range/CVSS4 correction; package source scopes and original publication preserved. No customer finding resolved.

  4. Revision 1 · created ·

    Published from osv record GHSA-9jh8-v38h-cvhr

Agent brief

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# Patch Next advisory brief: CVE-2026-63430

Advisory: ammonia: mXSS via MathML `annotation-xml` encoding strip
IDs: CVE-2026-63430, GHSA-9jh8-v38h-cvhr
Record: https://patchnext.com/advisories/cve-2026-63430 (revision 4, source checked 26 Oct 2026, 15:43 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 6.3)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: ammonia 4.1.3, 4.0.2, 3.3.2. A vendor fix does not mean a deployment is fixed.

## Affected components
- crates.io ammonia: affected < 3.3.2; >= 4.0.0, < 4.0.2; >= 4.1.0, < 4.1.3; fixed in 4.1.3, 4.0.2, 3.3.2

## Suggested inspection (read-only)
- crates.io ammonia: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on ammonia.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-9jh8-v38h-cvhr (OSV record GHSA-9jh8-v38h-cvhr lists affected ranges: crates.io ammonia >=0 =4.0.0 =4.1.0 <4.1.3 (fixed))
- Verified: https://github.com/advisories/GHSA-9jh8-v38h-cvhr (GitHub-reviewed advisory GHSA-9jh8-v38h-cvhr lists affected ranges: crates.io ammonia >=0 =4.0.0 =4.1.0 <4.1.3)

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