CVE-2026-44897: Mistune Heading ID Attribute Injection XSS

Severity: medium (CVSS 6.1 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.2% probability of exploitation in the next 30 days
Public exploit or detection template: yes
Published: 2026-05-26 · updated 2026-07-24
Weakness: CWE-79

Description

Mistune is a Python Markdown parser with renderers and plugins. Prior to 3.2.1, HTMLRenderer.heading() builds the opening <hN> tag by string-concatenating the id attribute value directly into the HTML — with no call to escape(), safe_entity(), or any other sanitisation function. A double-quote character " in the id value terminates the attribute, allowing an attacker to inject arbitrary additional attributes (event handlers, src=, href=, etc.) into the heading element. This vulnerability is fixed in 3.2.1.

Affected products

lepture mistune: before 3.2.1 — fixed in 3.2.1

How to fix

Upgrade to a fixed version: lepture mistune 3.2.1. Check the vendor advisory for mitigations.

References

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