What is CVE-2026-121437?

CVE-2026-121437 is a high-severity vulnerability in Lepture Mistune, classified under Inefficient Algorithmic Complexity. CVSS score: 7.5/10. Published 2026-11-03.

How severe is CVE-2026-121437?

High severity. CVSS v3 base score is 7.5 out of 10.

Vulnerability in Lepture Mistune

CVE-2026-121437

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.

Published 2026-11-03 · last modified 2026-11-03.

CVSS v3 metric

CVSS v3 base score 7.5 (High). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H.

EPSS exploit prediction

No EPSS score from FIRST.org yet. New CVEs usually get one within a few days; no score means no signal yet, not low risk. How EPSS works.

Affected products

Lepture Mistune

Weakness classification (CWE)

CWE-407 — Inefficient Algorithmic Complexity

References

github.com/lepture/mistune/security/advisories/GHSA-qp42-r9j6-83g9 (vendor-advisory)
github.com/lepture/mistune/commit/b4dd3284e52ec741ed9a227db29ade9017828e90 (patch)
www.vulncheck.com/advisories/mistune-3.3.0-before-3.3.4-quadratic-time-dos-via-… (third-party-advisory)

Frequently asked questions

What is CVE-2026-121437?: CVE-2026-121437 is a high-severity vulnerability in Lepture Mistune, classified under Inefficient Algorithmic Complexity. CVSS score: 7.5/10. Published 2026-11-03.
How severe is CVE-2026-121437?: High severity. CVSS v3 base score is 7.5 out of 10.