Summary
Mistune: XSS via unescaped class option in Admonition directive
Advisory details
In src/mistune/directives/admonition.py, the render_admonition() function concatenates the :class: option directly into the HTML class attribute without escaping (lines 63-68).
This allows attribute injection and XSS even when HTMLRenderer(escape=True) is used.
The directive name parameter is safe (validated against whitelist), but the class option comes from raw user input.
References
- https://github.com/advisories/GHSA-g97x-gvcm-x72h
- https://github.com/lepture/mistune/security/advisories/GHSA-g97x-gvcm-x72h
- https://nvd.nist.gov/vuln/detail/CVE-2026-59926
- https://github.com/lepture/mistune/commit/a3cb6e5655308797e8be021d6c7b5bab13cbace2
- https://github.com/lepture/mistune/releases/tag/v3.2.1
- https://github.com/pypa/advisory-database/tree/main/vulns/mistune/PYSEC-2026-2214.yaml
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