# CVE-2026-59926: lepture Mistune XSS in Admonition directive

Source: https://junglewise.ai/threats/cve-2026-59926-lepture-mistune-xss-in-admonition-directive  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-07-08. Updated: 2026-07-20  
License: CC BY 4.0 with a link back

CVE-2026-59926 · Severity: medium · CVSS 4 · Published 2026-07-08

Mistune is a popular Python library used to convert Markdown text into HTML. A security flaw in its 'Admonition' feature allows attackers to inject malicious scripts into web pages. If a user views a page containing specially crafted Markdown, the attacker could potentially steal session information or perform unauthorized actions on the user's behalf.

Technologies: [mistune (PyPI)](https://junglewise.ai/threats/technologies/mistune). Vendors: [PyPI](https://junglewise.ai/threats/vendors/pypi).

## Quick answers

**Is CVE-2026-59926 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-59926 being exploited in the wild.

**How severe is CVE-2026-59926?** Medium severity, with a CVSS score of 4, published 8 July 2026.

**Which products and versions are affected?** mistune (PyPI) < 3.3.0, ECOSYSTEM:introduced=0,fixed=3.3.0.

**Is there a fix for CVE-2026-59926?** Yes. mistune (PyPI): upgrade to 3.3.0 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-59926 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2026-59926?** Check whether you run mistune (PyPI) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## How to fix

- mistune (PyPI): upgrade to 3.3.0 or later

  ```sh
  pip install "mistune>=3.3.0"
  ```


## Technical details

A Cross-Site Scripting (XSS) vulnerability exists in Mistune's Admonition directive implementation within `src/mistune/directives/admonition.py`. The `render_admonition()` function fails to escape the `:class:` option before concatenating it into the HTML class attribute. This oversight allows for attribute injection and XSS even when the `HTMLRenderer` is configured with `escape=True`. An attacker can exploit this by providing raw user input in the class option of an Admonition directive. The issue is resolved in version 3.3.0.

## Affected products

- lepture mistune < 3.3.0

## Identifiers

- CVE-2026-59926
- GHSA-g97x-gvcm-x72h
- PYSEC-2026-2214

## Timeline

- 2026-06-21: disclosed
- 2026-07-08: advisory: NVD publication
- 2026-07-20: advisory: GitHub Advisory published

## References

- [https://api.github.com/users/sergeykochanov](https://api.github.com/users/sergeykochanov)
- [https://github.com/sergeykochanov](https://github.com/sergeykochanov)
- [https://api.github.com/users/sergeykochanov/gists%7B/gist_id%7D](https://api.github.com/users/sergeykochanov/gists%7B/gist_id%7D)
- [https://api.github.com/users/sergeykochanov/repos](https://api.github.com/users/sergeykochanov/repos)
- [https://avatars.githubusercontent.com/u/47022537?v=4](https://avatars.githubusercontent.com/u/47022537?v=4)
- [https://api.github.com/users/sergeykochanov/events%7B/privacy%7D](https://api.github.com/users/sergeykochanov/events%7B/privacy%7D)

## Related threats

- [Mistune path traversal and XSS in Include directive](https://junglewise.ai/threats/duplicate-advisory-mistune-path-traversal-and-xss-in-include-directive-bf25c187)
- [Mistune path traversal and XSS in Include directive](https://junglewise.ai/threats/cve-2026-115355-mistune-before-3-3-0-contains-a-path-traversal-vulnerability-in)
- [Mistune uncontrolled recursion in HTML rendering](https://junglewise.ai/threats/cve-2026-76098-mistune-uncontrolled-recursion-in-html-rendering)
- [lepture Mistune predictable ID generation in TOC plugin](https://junglewise.ai/threats/cve-2026-59930-lepture-mistune-predictable-id-generation-in-toc-plugin)
- [lepture mistune XSS via incomplete URL scheme filtering in HTML renderer](https://junglewise.ai/threats/cve-2026-59929-lepture-mistune-xss-via-incomplete-url-scheme-filtering-in-html)