XSS in Lepture Mistune
CVE-2026-44899
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.
Vulnerability class: XSS (Cross-Site Scripting)
Published · last modified .
CVSS v3 metric
CVSS v3 base score 4.7 (Medium). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:N/A:N.
EPSS exploit prediction
EPSS: 0.003 (18.2th percentile), scored .
Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 18th percentile — 18.2% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.
EPSS over last 30 days · oldest: 0.002 · newest: 0.003 · change: +0.000
Affected products
- Lepture Mistune — versions < 3.2.1
- Mistune Project
Weakness classification (CWE)
References
- github.com/lepture/mistune/releases/tag/v3.2.1 (Product, Release Notes)
- github.com/lepture/mistune/security/advisories/GHSA-ccfx-mfmx-2fx9 (Exploit, Vendor Advisory)
Frequently asked questions
- What is CVE-2026-44899?
- CVE-2026-44899 is a medium-severity vulnerability in Lepture Mistune, classified under Cross-site Scripting. CVSS score: 4.7/10. Published 2026-05-26.
- How severe is CVE-2026-44899?
- Medium severity. CVSS v3 base score is 4.7 out of 10.