CVE-2025-66400: mdast-util-to-hast unsanitized class attribute

Severity: medium (CVSS 5.3 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.3% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2025-12-01 · updated 2026-10-18
Weakness: CWE-20

Description

mdast-util-to-hast is an mdast utility to transform to hast. From 13.0.0 to before 13.2.1, 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. This vulnerability is fixed in 13.2.1.

Affected products

syntax-tree mdast-util-to-hast: from 13.0.0 before 13.2.1 — fixed in 13.2.1

How to fix

Upgrade to a fixed version: syntax-tree mdast-util-to-hast 13.2.1. Check the vendor advisory for mitigations.

References

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/security/advisories/GHSA-4fh9-h7wg-q85m

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