CVE-2026-59926 Mistune: XSS via unescaped class option in Admonition directive
Attackers can inject malicious scripts via unescaped class options in Admonition directives, affecting applications using the Mistune Markdown parser.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- A user has to click or open something
- Fix
- Available
What is CVE-2026-59926?
CVE-2026-59926 is a medium-severity cross-site scripting vulnerability in lepture mistune, published on 8 Jul 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.
Official description
Mistune is a Python Markdown parser with renderers and plugins. Prior to 3.2.1, render_admonition() in src/mistune/directives/admonition.py concatenates the Admonition directive :class: option into the HTML class attribute without escaping, allowing attribute injection and cross-site scripting even when HTMLRenderer escape mode is enabled. This issue is fixed in version 3.2.1.
Weakness: CWE-79
Which versions are affected by CVE-2026-59926?
3 open source packages ship an affected version, including mistune. It affects 1 product.
| Package | Affected | Fixed in |
|---|---|---|
mistune PyPI | All versions | 3.3.0 |
mistune PyPI | All versions | 3.3.0 |
mistune PyPI | All versions | 3.3.0 |
Products include Mistune Project Mistune.
How do I fix CVE-2026-59926?
Upgrade mistune to 3.3.0 or later.
Escape output for the context it lands in and set a strict Content Security Policy. Upgrading the affected component is the real fix. More about Cross-site scripting.
Is CVE-2026-59926 being exploited?
There is no known exploitation of CVE-2026-59926 in the wild and no public exploit code.
Other vulnerabilities in lepture mistune
- CVE-2026-59922 Mistune plugins/formatting: quadratic-time parsing on long runs of `~~x~~`, `==x==`, and `^^x^^` markers (strikethrough / mark / insert)
- CVE-2026-59925 inline_parser: quadratic-time parsing on long runs of `**x**` and `***x***` emphasis pairs
- CVE-2026-59928 Mistune block_parser: quadratic-time parsing on long lists of repeated reference-link definitions
- CVE-2026-59923 Mistune: XSS via percent-encoded javascript URI bypass in safe_url()
- CVE-2026-59929 Mistune renderers/html.safe_url: HARMFUL_PROTOCOLS list misses legacy and chained schemes that historically chain to `javascript:` execution
Sources
Also known as AZL-92214, DEBIAN-CVE-2026-59926, ECHO-4c0d-898e-3e31, GHSA-g97x-gvcm-x72h, PYSEC-2026-2214, UBUNTU-CVE-2026-59926.