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CVE-2026-44899: Mistune Image Directive CSS Injection Vulnerability

• 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, the Image directive plugin validates the :width: and :height: options with a regex compiled as _num_re = re.compile(r"^\d+(?:\.\d*)?"). When the validated value is not a plain integer, render_block_image() inserts it directly into a style="width:...;" or style="height:...;" attribute. Because the value was accepted by the prefix-only regex, any CSS after the leading digits reaches the style= attribute verbatim and without escaping. 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

• https://github.com/lepture/mistune/releases/tag/v3.2.1
• https://github.com/lepture/mistune/security/advisories/GHSA-ccfx-mfmx-2fx9
• https://github.com/lepture/mistune/security/advisories/GHSA-ccfx-mfmx-2fx9

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