NVD security advisoryHigh
High advisory for Mistune from 3.3.0
CVE-2026-121437 · Reported 3 Nov 2026
Full advisory details, affected packages, severity context, and original source.
Affected technology
Mistune from 3.3.0Affected product
Advisory overview
Mistune from 3.3.0 before 3.3.4 contains a denial of service vulnerability that allows remote attackers to exhaust CPU through quadratic-time emphasis delimiter processing. Attackers can submit small markdown documents with repeated patterns like "*a_b" to trigger an O(n²) index-fixup loop, stalling workers for seconds.
Related references
- https://github.com/lepture/mistune/commit/b4dd3284e52ec741ed9a227db29ade9017828e90
- https://github.com/lepture/mistune/security/advisories/GHSA-qp42-r9j6-83g9
- https://www.vulncheck.com/advisories/mistune-3.3.0-before-3.3.4-quadratic-time-dos-via-process-emphasis-delimiters
- https://github.com/lepture/mistune/security/advisories/GHSA-qp42-r9j6-83g9