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Vulnerability record · CVE-2026-44899 · published 26 May 2026

CVE-2026-44899: Mistune project mistune cross-site scripting vulnerability

Mistune Project · Mistune

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.

6.1 CVSS 3.1 Medium EPSS 0.27% · top 82.5% CWE-79 · Cross-site scripting
6.1CVSS 3.1 base score
0.27%EPSS exploitation probability, 30 days
NoNot in CISA KEV
1Affected product versions listed by NVD
3References, 2 tagged exploit
24 Jul 2026Last modified by NVD

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.

CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N

Affected products

1 vulnerable configurations from NVD's CPE data, grouped by vendor and product.

References

LinkTags
https://github.com/lepture/mistune/releases/tag/v3.2.1 ProductRelease Notes
https://github.com/lepture/mistune/security/advisories/GHSA-ccfx-mfmx-2fx9 ExploitVendor Advisory
https://github.com/lepture/mistune/security/advisories/GHSA-ccfx-mfmx-2fx9 ExploitVendor Advisory

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Same products first, then exploited flaws of the same weakness class.

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Source: NIST National Vulnerability Database (record CVE-2026-44899), CISA KEV, FIRST EPSS (scores of 2026-09-27). This page is refreshed as NVD updates the record.