RUSTSEC-2026-0193 (GHSA-9JH8-V38H-CVHR)

Vulnerability from osv_rustsec – Published: 2026-06-30 12:00 – Updated: 2026-07-17 05:23 – Source website
VLAI
Summary
mXSS in ammonia via MathML `annotation-xml` encoding strip
Details

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.

Discovered by: Ivan Ivančić · Date: 2026-06-29 · Found via local differential analysis and source review of ammonia's sanitisation pipeline; no third-party systems were tested.

References
To clipboard 

{
  "affected": [
    {
      "database_specific": {
        "categories": [
          "format-injection"
        ],
        "cvss": null,
        "informational": null
      },
      "ecosystem_specific": {
        "affected_functions": null,
        "affects": {
          "arch": [],
          "functions": [],
          "os": []
        }
      },
      "package": {
        "ecosystem": "crates.io",
        "name": "ammonia",
        "purl": "pkg:cargo/ammonia"
      },
      "ranges": [
        {
          "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"
            }
          ],
          "type": "SEMVER"
        }
      ],
      "versions": []
    }
  ],
  "aliases": [
    "GHSA-9jh8-v38h-cvhr",
    "CVE-2026-63430"
  ],
  "database_specific": {
    "license": "CC0-1.0"
  },
  "details": "If a certain set of MathML tags are enabled, an attacker can inject arbitrary JavaScript code into the user\u0027s browser.\n\nThe `annotation-xml` tag has slightly different behavior than the other \"integration point\"\ntags in MathML and SVG, but ammonia didn\u0027t handle it, so it didn\u0027t correctly\nstrip the namespace-incompatible tags.\n\nThis vulnerability only has an effect when the `math` and `annotation-xml` tags\nare both enabled, but the `encoding` attribute is disabled, because it relies\non the following sequence of steps:\n\n1. User writes code like `\u003cmath\u003e\u003cannotation-xml encoding=\"text/html\"\u003e\u003cgadget\u003e\u003c/annotation-xml\u003e\u003c/math\u003e`.\n2. Namespace filtering checks the DOM, and it passes. `\u003cgadget\u003e` is parsed as HTML.\n3. Attribute filter strips it down to `\u003cmath\u003e\u003cannotation-xml\u003e\u003cgadget\u003e\u003c/annotation-xml\u003e\u003c/math\u003e`. Because the encoding attribute is gone, `\u003cgadget\u003e` is now parsed as MathML.\n4. The gadget is written in such a way that it exploits the parsing differences between HTML and MathML.\n\nAdditionally, the gadget can only be written using a tag that is parsed as raw text in HTML.\nThese [elements] are:\n\n* title\n* textarea\n* xmp\n* iframe\n* noembed\n* noframes\n* plaintext\n* noscript\n* style\n* script\n\nApplications that do not explicitly allow any of these tags should not be affected, since none are allowed by default.\n\n[elements]: https://github.com/servo/html5ever/blob/045a0378f2b0f8d4a350793899cf722a2a9b3d11/html5ever/src/tree_builder/rules.rs\n\n---\n\n**Discovered by:** [Ivan Ivan\u010di\u0107](https://ivan09999.github.io/ammonia-mxss) \u00b7 **Date:** 2026-06-29 \u00b7 Found via local differential analysis and source review of ammonia\u0027s sanitisation pipeline; no third-party systems were tested.",
  "id": "RUSTSEC-2026-0193",
  "modified": "2026-07-17T05:23:11Z",
  "published": "2026-06-30T12:00:00Z",
  "references": [
    {
      "type": "PACKAGE",
      "url": "https://crates.io/crates/ammonia"
    },
    {
      "type": "ADVISORY",
      "url": "https://rustsec.org/advisories/RUSTSEC-2026-0193.html"
    }
  ],
  "related": [],
  "severity": [],
  "summary": "mXSS in ammonia via MathML `annotation-xml` encoding strip"
}



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