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CVE-2026-63430 medium CVSS 4.7

Cross-Site Scripting (XSS) in ammonia (CVE-2026-63430)

Summary

Cross-site scripting in ammonia (CVE-2026-63430). Risk of unauthorized operations or information disclosure. Exploitable via math. Mitigation: upgrade to 3.3.2, 4.0.2, 4.1.3 or later.

Original description

The description as published by the source.

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 ``.
  2. Namespace filtering checks the DOM, and it passes. `` is parsed as HTML.
  3. Attribute filter strips it down to . Because the encoding attribute is gone, 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.

Discovered by: Ivan Ivančić · Date: 2026-06-29 · Found via local differential analysis and source review of ammonia's sanitisation pipel…

Source: OSV.dev (Open Source Vulnerabilities) NVD GitHub Advisory OSV

Auto-generated summary

Assembled by SecPulse from the source data (CVSS, CWE, affected versions) using templates — no AI involved. Check the original description and references below for exact details.

A vulnerability tracked as CVE-2026-63430 has been found in ammonia.
Attackers can target a specific entry point like math over the network to misuse the product.
Risk of unauthorized operations or information disclosure. CVSS score: 4.7/10.
What to do: upgrade ammonia to 3.3.2, 4.0.2, 4.1.3 or later.
If unsure, ask your IT team or search "ammonia CVE-2026-63430" on the vendor's site.

CVE-2026-63430 (ammonia) — CWE-79 / CVSS v3 4.7
Attack vector: remote (network-reachable) / unauthenticated
Attack surface: math / encoding
Patched: 3.3.2, 4.0.2, 4.1.3 — apply immediately
Plan: 1) Audit SBOM/dependencies, 2) Stage→prod upgrade, 3) Add WAF/proxy monitoring on affected endpoints, 4) Hunt IOCs in logs.
Refs: see the GHSA / vendor advisory / patched release linked on this page.

❓ What is the problem

Cross-site scripting (XSS) (CWE-79) exists in ammonia. Attackers reach the vulnerable code path via math without authentication.

📍 Affected scope

Check the vendor advisory for the affected versions of ammonia. Attack surface: math / encoding.

🔥 Severity

Severity: Medium (CVSS 4.7/10). Risk of unauthorized operations or information disclosure.

🔧 How to fix

Update to 3.3.2, 4.0.2, 4.1.3.

🛡️ Workaround

Until the patch is applied: disable the affected feature, apply WAF rules, or restrict access via network ACLs.

🔍 Detection

Search webserver/proxy logs for unusual request patterns matching this CVE's known IOCs. Run grep -r 'ammonia' . against your dependency files (package-lock.json, requirements.txt, go.sum) to find affected services.

Response actions

Concrete steps and command examples for SOC/SRE teams to execute in order

  1. 1
    Identify exposure identify 
    grep -r 'ammonia' . | grep -v node_modules

    リポジトリと本番環境の依存ファイル (package-lock.json / requirements.txt / go.sum / Gemfile.lock 等) で ammonia を grep し、稼働しているサービス・バージョンを把握する。

  2. 2
    Apply patch patch 
    Upgrade ammonia to 3.3.2, 4.0.2, 4.1.3

    ステージング環境で 3.3.2, 4.0.2, 4.1.3 に上げて回帰テスト → 本番反映。回帰テストはアプリの主要ハッピーパスを含めること。

  3. 3
    Post-deployment verification verify 
    Confirm patched version is live in production

    パッチ適用後、ステージングで PoC または同等の悪用パターンを再現して脆弱性が閉じたことを確認。本番ではアクセスログ・WAF でアラート再発が無いか継続監視。

Affected packages

crates.io ammonia
[{"type":"SEMVER","events":[{"introduced":"0.0.0-0"},{"fixed":"3.3.2"},{"introduced":"4.0.0"},{"fixed":"4.0.2"},{"introduced":"4.1.0"},{"fixed":"4.1.3"}]}]

Fix status by distribution

Fixed packages published by each distribution for this CVE

References

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