mdast-util-to-hast has unsanitized class attribute
Impact
Multiple (unprefixed) classnames could be added in markdown source by using character references.
This could make rendered user supplied markdown code elements appear like the rest of the page.
The following markdown:
```js xss
```Would create <pre><code class="language-js xss"></code></pre>
If your page then applied .xss classes (or listeners in JS), those apply to this element.
For more info see https://github.com/ChALkeR/notes/blob/master/Improper-markup-sanitization.md#unsanitized-class-attribute
Patches
The bug was patched. When using regular semver, run npm install. For exact ranges, make sure to use 13.2.1.
Workarounds
Update.
References
Affected software
Click a technology to see all vulnerabilities affecting it.
| Platform | Package | Vulnerable versions | Fixed in |
|---|---|---|---|
| npm package | mdast-util-to-hast | >= 13.0.0, < 13.2.1 | 13.2.1 |
References
- https://github.com/syntax-tree/mdast-util-to-hast/security/advisories/GHSA-4fh9-h7wg-q85m
- https://nvd.nist.gov/vuln/detail/CVE-2025-66400
- https://github.com/syntax-tree/mdast-util-to-hast/commit/6fc783ae6abdeb798fd5a68e7f3f21411dde7403
- https://github.com/syntax-tree/mdast-util-to-hast/commit/ab3a79570a1afbfa7efef5d4a0cd9b5caafbc5d7
- https://github.com/syntax-tree/mdast-util-to-hast
- CVSS vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N- Weakness (CWE)
CWE-20, CWE-915- Platform
- npm
- Package
mdast-util-to-hast- Disclosed
- 02.12.2025
- Updated
- 11.10.2026 10:22
Sources and licenses
OSV / GitHub Advisory Database - GHSA-4fh9-h7wg-q85m - CC BY 4.0
Vulnerability history: mdast-util-to-hast
All published vulnerabilities of this package (npm package), newest first. Shows whether this is a one-off slip or a package that needs watching.
This is the only known vulnerability of this package.
Know which vulnerabilities affect your application
With the DockRay integration and software inventory enabled, DockRay checks reported component versions against known vulnerabilities and notifies you about confirmed matches. Coverage depends on the supported technology and available advisory data.