MediumCVE-2026-63430

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

    Published Last modified Sources: OSV, Hersteller, CVE.org

    What is affected, what to do

    Update Rust-ammonia Ammonia to version 3.3.2.

    Affected
    • Rust-ammonia Ammonia gte 4.0.0, < 4.0.2..*
    • Rust-ammonia Ammonia lt *..3.3.2
    Fixed in
    • Rust-ammonia Ammonia 3.3.2

    Not sure whether and how you are affected? Ask a question about this vulnerability

    Metrics

    6.3

    Priority

    XIEM Effective CVSS: highest available CVSS rating from NVD and vendor data.

    Source: Vendor (CNA), CVSS v4

    6.3

    CVSS v4

    Severity Medium on the scale from 0 to 10.

    Source: OSV, Hersteller, CVE.org

    0.6 %

    EPSS score

    Low — model estimates < 1% exploitation likelihood.

    Source: FIRST EPSS, percentile 46.2

    Show all metrics
    Severity
    Medium
    Priority (0–325)

    The priority score is a consolidated metric that condenses several individual signals into a single, action-guiding ranking: technical severity (CVSS), exploitation likelihood (EPSS), known-exploited status (KEV) and the context of the affected asset. The aim is to answer the only truly important question in vulnerability management: what first?

    Read in glossary
    78.56 Methodology
    Exploit maturity

    An exploit is code or a technique that specifically uses a concrete vulnerability to force unintended behaviour — such as executing code or escalating privileges. It is the tool that turns a theoretical flaw into a practical attack.

    Read in glossary
    no public PoC known
    XIEM Effective CVSS

    XIEM® is the tiered managed-security operating model from NEOSEC and a registered trademark of NEOSEC GmbH. It extends classic SIEM — the central collection, normalisation and correlation of security-relevant events — with endpoint forensics, network visibility and deception technology, and consolidates them into three building tiers: Sentry, Orchestrate and Command.

    Read in glossary
    6.3 Methodology
    CVSS v4

    CVSS is the globally established, open standard for assessing the technical severity of a vulnerability on a scale from 0 to 10. It describes how easily a flaw can be exploited and what damage it can cause — but not how likely its actual exploitation is. CVSS and the complementary EPSS deliberately answer different questions.

    Read in glossary
    6.3
    EPSS score

    The EPSS score is the absolute model output for an individual vulnerability: the estimated probability of real-world exploitation within the next 30 days, expressed as a value between 0 and 1 (i.e. 0 % to 100 %). A score of 0.42 means „the model estimates a 42 % chance of real-world exploitation within 30 days“.

    Read in glossary
    0.6 %
    Low — model estimates < 1% exploitation likelihood.
    EPSS percentile

    The EPSS percentile is the relative position of a vulnerability within all EPSS-scored CVEs, expressed as a value between 0 and 1. A percentile of 0.90 means „this CVE has a higher EPSS score than 90 % of all other scored CVEs“. Unlike the EPSS score, it is not a probability — but a positioning within a ranking.

    Read in glossary
    46.2
    High — this CVE ranks above the median of all CVEs scored today (rank ≥ 36%).
    KEV status

    The KEV catalogue is a list maintained by the US authority CISA of vulnerabilities for which active exploitation in the wild has been confirmed. It answers not the question „how severe?“ but „is it already being attacked?“ — thereby sharply prioritising patching.

    Read in glossary
    —
    Published
    CISA assessment (SSVC)
    Exploitation: proof of concept · automatable · Technical impact: partial
    Source: CISA Vulnrichment, as of 15 October 2026
    CWE

    CWE is a catalogue of typical weakness types in software and hardware — such as flawed input validation, unsafe memory handling or poor authentication. Where a CVE names a concrete vulnerability in a concrete product, a CWE describes the underlying class of defect.

    Read in glossary
    CWE-79

    Detection rules

    No NEOSEC detection rule is available for this vulnerability yet.

    Classification under NIS2 and CRA

    Rule-based classification from exploitation status and severity. It does not replace a case-by-case assessment.

    NIS2

    NIS2 is EU Directive (EU) 2022/2555 to raise the level of cybersecurity across the Union. It substantially broadens obligations compared with the first NIS directive: more sectors, mandatory risk management, staged reporting duties and personal accountability of senior management.

    Read in glossary
    Handle the vulnerability in regular vulnerability handling under Art. 21(2)(e) NIS2.
    CRA

    The CRA is EU Regulation (EU) 2024/2847 setting horizontal cybersecurity requirements for „products with digital elements“ — from industrial controllers and software to connected consumer devices. For the first time manufacturers must ensure security across the entire product lifecycle, handle vulnerabilities and provide security updates.

    Read in glossary
    From 11.12.2027, Annex I Part II CRA requires manufacturers of affected products with digital elements to handle vulnerabilities without delay and provide security updates.
    ISO/IEC 27001

    ISO/IEC 27001 is the internationally leading standard for an information security management system (ISMS). It describes a risk-based, process-oriented framework to systematically plan, implement, review and improve information security.

    Read in glossary
    Control A.8.8, management of technical vulnerabilities: assess relevance to your systems, plan remediation and keep evidence.

    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:

    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 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

    Workarounds

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

    Source: OSV (Open Source Vulnerabilities), GitHub Security Advisories, cvelistv5, NVD (NIST), CVE.org (MITRE)

    Affected products

    The following versions are affected. Older releases of the same product line are also vulnerable unless stated otherwise.

    Affected products
    Affected versionsFixed in
    rust-ammoniaammonia
    • ≥ 4.0.0, < 4.0.2
    • < 3.3.2
    • —
    • 3.3.2
    crates.ioammonia
    • 4.1.0
    • 4.0.0
    • —
    • 4.1.3
    • 4.0.2
    • 3.3.2
    rustammonia
    • >= 4.0.0, < 4.0.2
    • < 3.3.2
    • >= 4.1.0, < 4.1.3
    • 4.0.2
    • 3.3.2
    • 4.1.3

    Vendors and products

    Weakness classes (CWE)

    • CWE-79Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')

      Base

      The product does not neutralize or incorrectly neutralizes user-controllable input before it is placed in output that is used as a web page that is served to other users.

      Entry at MITRE CWE-79

    References & sources

    • crates.iocrates/ammoniapackage
    • rustsec.orgadvisories/RUSTSEC-2026-0193.htmladvisory, x_refsource_MISC, web
    • github.comrust-ammonia/ammonia/security/advisories/GHSA-9jh8-v38h-cvhrx_refsource_CONFIRM, web
    • github.comrust-ammonia/ammonia/pull/243x_refsource_MISC, web
    • github.comrust-ammonia/ammonia/pull/244x_refsource_MISC, web
    • github.comrust-ammonia/ammonia/pull/245x_refsource_MISC, web
    • github.comrust-ammonia/ammonia/commit/004bf2cee4288f74a6ee619d589e3e90c24…x_refsource_MISC, web
    • github.comrust-ammonia/ammonia/commit/7ac3fd3145236f28857eeecbadb544c8659…x_refsource_MISC, web
    • github.comrust-ammonia/ammonia/commit/b51bf4f9749ed3cd518330105f20a2c39c9…x_refsource_MISC, web
    • github.comrust-ammonia/ammonia/releases/tag/v3.3.2x_refsource_MISC, web
    • github.comrust-ammonia/ammonia/releases/tag/v4.0.2x_refsource_MISC, web
    • github.comrust-ammonia/ammonia/releases/tag/v4.1.3x_refsource_MISC, web
    • ivan09999.github.ioammonia-mxssx_refsource_MISC, web
    • NVD (NIST)vuln/detail/CVE-2026-63430advisory
    • github.comrust-ammonia/ammoniapackage

    Changes to this CVE

    Who changed this CVE and when: NVD (analysis, affected products, CVSS), the CVE publisher (CNA) and additional providers such as CISA (ADP).

    1. Record modifiedCISA ADP

      CISA assessment (SSVC): exploitation: proof of concept, automatable: yes

      Show 2 original changes
      Individual changes of this event as published by NVD
      TypeActionBeforeAfter
      Referenceadded
      —
      https://github.com/rust-ammonia/ammonia/security/advisories/GHSA-9jh8-v38h-cvhr
      SSVC assessmentadded
      —
      {"id":"CVE-2026-63430","role":"CISA Coordinator","options":[{"exploitation":"poc"},{"automatable":"yes"},{"technicalImpact":"partial"}],"version":"2.0.3","timestamp":"2026-10-15T20:24:18.831464Z"}

      Source identifier: 134c704f-9b21-4f2e-91b3-4a467353bcc0

    2. ReceivedGitHub

      CVSS V4.0: AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:N/SC:L/SI:L/SA:N

      Weakness type: CWE-79

      Description added or changed

      Show 16 original changes
      Individual changes of this event as published by NVD
      TypeActionBeforeAfter
      Descriptionadded
      —
      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.
      CVSSadded
      —
      AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:N/SC:L/SI:L/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X
      Weakness type (CWE)added
      —
      CWE-79
      Affected versions (publisher)added
      —
      New affected value received. <a href="https://github.com/CVEProject/cvelistV5/blob/main/cves/2026/63xxx/CVE-2026-63430.json">CVE-2026-63430</a>
      Referenceadded
      —
      https://github.com/rust-ammonia/ammonia/commit/004bf2cee4288f74a6ee619d589e3e90c24c09ca
      Referenceadded
      —
      https://github.com/rust-ammonia/ammonia/commit/7ac3fd3145236f28857eeecbadb544c8659cba31
      Referenceadded
      —
      https://github.com/rust-ammonia/ammonia/commit/b51bf4f9749ed3cd518330105f20a2c39c9ee274
      Referenceadded
      —
      https://github.com/rust-ammonia/ammonia/pull/243
      Referenceadded
      —
      https://github.com/rust-ammonia/ammonia/pull/244
      Referenceadded
      —
      https://github.com/rust-ammonia/ammonia/pull/245
      Referenceadded
      —
      https://github.com/rust-ammonia/ammonia/releases/tag/v3.3.2
      Referenceadded
      —
      https://github.com/rust-ammonia/ammonia/releases/tag/v4.0.2
      Referenceadded
      —
      https://github.com/rust-ammonia/ammonia/releases/tag/v4.1.3
      Referenceadded
      —
      https://github.com/rust-ammonia/ammonia/security/advisories/GHSA-9jh8-v38h-cvhr
      Referenceadded
      —
      https://ivan09999.github.io/ammonia-mxss
      Referenceadded
      —
      https://rustsec.org/advisories/RUSTSEC-2026-0193.html

      Source identifier: security-advisories@github.com

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