XSS in Lepture Mistune
CVE-2026-44898
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.
Vulnerability class: XSS (Cross-Site Scripting)
Published · last modified .
CVSS v3 metric
CVSS v3 base score 6.1 (Medium). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/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-6269-cqxg-mhhv (Exploit, Vendor Advisory)
Frequently asked questions
- What is CVE-2026-44898?
- CVE-2026-44898 is a medium-severity vulnerability in Lepture Mistune, classified under Cross-site Scripting. CVSS score: 6.1/10. Published 2026-05-26.
- How severe is CVE-2026-44898?
- Medium severity. CVSS v3 base score is 6.1 out of 10.