Bug 2481726 (CVE-2026-44898)

Summary:	CVE-2026-44898 mistune: Mistune: Arbitrary code execution via HTML injection in table of contents rendering
Product:	[Other] Security Response	Reporter:	OSIDB Bzimport <bzimport>
Component:	vulnerability	Assignee:	Product Security DevOps Team <prodsec-dev>
Status:	NEW ---	QA Contact:
Severity:	medium	Docs Contact:
Priority:	medium
Version:	unspecified	CC:	anthomas, ehelms, ggainey, jkoehler, juwatts, lphiri, mhulan, nmoumoul, osousa, pcreech, rchan, rjohnson, smallamp, tmalecek
Target Milestone:	---	Keywords:	Security
Target Release:	---
Hardware:	All
OS:	Linux
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Doc Text:	A flaw was found in Mistune, a Python Markdown parser. This vulnerability occurs in the `render_toc_ul()` function, which is responsible for building a table-of-contents. An attacker can craft malicious heading text that, when processed, allows for the injection of arbitrary HTML tags, including script blocks, into the rendered table of contents. This could lead to client-side arbitrary code execution or information disclosure.	Story Points:	---
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oVirt Team:	---	RHEL 7.3 requirements from Atomic Host:
Cloudforms Team:	---	Target Upstream Version:
Embargoed:
Bug Depends On:	2489783, 2489784, 2489782
Bug Blocks:

Description OSIDB Bzimport 2026-05-26 22:02:11 UTC

Mistune is a Python Markdown parser with renderers and plugins. Prior to 3.2.1, render_toc_ul() builds a <ul> table-of-contents tree from a list of (level, id, text) tuples. Both the id value (used as href="#<id>") and the text value (used as the visible link label) are inserted into <a> tags via a plain Python format string — with no HTML escaping applied to either value. When heading IDs are derived from user-supplied heading text (the standard use-case for readable slug anchors), an attacker can craft a heading whose text breaks out of the href="#..." attribute context, injecting arbitrary HTML tags including <script> blocks directly into the rendered TOC. This vulnerability is fixed in 3.2.1.