How severe is CVE-2026-63430?

No CVSS score has been assigned to CVE-2026-63430 yet. Review the advisory details and affected package list to assess your exposure.

Which packages are affected by CVE-2026-63430?

CVE-2026-63430 affects the following packages: ammonia (crates.io), ammonia (crates.io), ammonia (crates.io). Ecosystems affected: crates.io.

How do I fix CVE-2026-63430?

Update ammonia to 3.3.2 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-63430 is resolved across your whole dependency graph.

How do I detect CVE-2026-63430 in my crates.io dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for ammonia, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

How do I mitigate CVE-2026-63430 if there is no patch (or I can't update yet)?

Escape or sanitise the affected output on the server side rather than relying on client-side filtering, and add a Content-Security-Policy that blocks inline script execution so injected markup cannot run even if it reaches the page.

Is CVE-2026-63430 actively exploited in the wild?

No public exploit code has been indexed for CVE-2026-63430 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What is the EPSS score for CVE-2026-63430?

CVE-2026-63430 has an EPSS (Exploit Prediction Scoring System) score of 0.6%, placing it in the 45th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is CVE-2026-63430?

CVE-2026-63430 is classified as Cross-site Scripting (XSS) (CWE-79). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation. This is a high-impact weakness class that often enables remote code execution or data exposure.

When was CVE-2026-63430 published, and has it been updated?

CVE-2026-63430 was published on October 15, 2026 and was last updated on October 21, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

🦀 crates.io

Not in CISA KEV

CVE-2026-63430 — ammonia

Fix: rust-ammonia/ammonia@004bf2c

CVE-2026-63430 is a Cross-site Scripting (XSS) vulnerability in ammonia. A fix is available for ammonia — see the affected versions and patch details below.

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

Also known asGHSA-9jh8-v38h-cvhrRUSTSEC-2026-0193

Published

Oct 15, 2026

Updated

Oct 21, 2026

Affected

3 pkgs

Patched

3 / 3

Exploits

None indexed

Exploitation data as of Oct 21, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

Exploitation Status

Proof-of-concept exploit code exists

CISA’s SSVC triage found public proof-of-concept exploit code for this CVE, though no confirmed active exploitation.
CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.

Exploitation and automatability from CISA’s SSVC triage for CVE-2026-63430.

EPSS Exploitation Probability

via FIRST.org ↗

0.6%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs45th percentile — riskier than 45% of all scored CVEsHighest risk

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

Real-World Exposure

3 pkgs affected

🦀ammonia🦀ammonia🦀ammonia

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects crates.io packages — download data is not available via public APIs for these ecosystems.

Description

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:

User writes code like <math><annotation-xml encoding="text/html"><gadget></annotation-xml></math>.
Namespace filtering checks the DOM, and it passes. <gadget> is parsed as HTML.
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.
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.

Impact

Low: Requires both MathML and at least one raw-text HTML tag to be enabled, and most websites don't enable either of those.

Patches

https://github.com/rust-ammonia/ammonia/pull/245 (released in 4.1.3)
https://github.com/rust-ammonia/ammonia/pull/244 (released in 4.0.2)
https://github.com/rust-ammonia/ammonia/pull/243 (released in 3.3.2)

Workarounds

Call <code>builder.add_clean_content_tags("annotation-xml")</code> to remove the annotation-xml element and all of its children, including the XSS gadget.

Affected Packages

3 total 3 fixed

Ecosystem	Package	Vulnerable range	Fix
🦀crates.io	`ammonia`	all versions	3.3.2`cargo update -p ammonia --precise 3.3.2`
🦀crates.io	`ammonia`	≥ 4.0.0&&< 4.0.2	4.0.2`cargo update -p ammonia --precise 4.0.2`
🦀crates.io	`ammonia`	≥ 4.1.0&&< 4.1.3	4.1.3`cargo update -p ammonia --precise 4.1.3`

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for ammonia, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update ammonia to 3.3.2 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-63430 is resolved across your whole dependency graph.
Workarounds
Escape or sanitise the affected output on the server side rather than relying on client-side filtering, and add a Content-Security-Policy that blocks inline script execution so injected markup cannot run even if it reaches the page.

Frequently Asked Questions

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

O3 Security · Impact-Aware SCA

Is CVE-2026-63430 in your dependencies?

Find it across crates.io, including transitive dependencies.

Scan my dependencies How O3 SCA works