CVE-2026-121437

High

Description

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.

SeverityHigh

CVSS7.5

Exploit probability—

ProductLepture Mistune

PublishedNov 3, 2026

First seenNov 4, 2026

Weakness typeCWE-407CWE-407

How to fix

Update

lepture mistune3.3.4 or later

Patches

github.com/lepture/mistune/commit/b4dd3284e52ec741ed9a227db29ade9017828e90

Vendor advisories

github.com/lepture/mistune/security/advisories/GHSA-qp42-r9j6-83g9

Affected products

Product	Affected versions	Fixed in
lepture mistune	3.3.0 ≤ v < 3.3.4	3.3.4

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