What is CVE-2026-121438?

CVE-2026-121438 is a medium-severity vulnerability in Lepture Mistune, classified under Uncontrolled Resource Consumption. CVSS score: 5.3/10. Published 2026-11-03.

How severe is CVE-2026-121438?

Medium severity. CVSS v3 base score is 5.3 out of 10.

Resource exhaustion in Lepture Mistune

CVE-2026-121438

Mistune before 3.3.3 contains a denial of service vulnerability in InlineParser.parse_link that allows attackers to cause quadratic CPU growth by supplying balanced nested brackets. Attackers can submit crafted Markdown with thousands of nested bracket pairs not forming a valid link, triggering repeated _contains_nested_link scans that exhaust CPU in parsing applications.

Vulnerability class: DoS (Denial of Service)

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

CVSS v3 metric

CVSS v3 base score 5.3 (Medium). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L.

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-400 — Uncontrolled Resource Consumption

References

github.com/lepture/mistune/security/advisories/GHSA-47p6-w2rc-5qf7 (vendor-advisory)
github.com/lepture/mistune/commit/fe02f40faf091f86da9ed62c865f31e11da111d7 (patch)
www.vulncheck.com/advisories/mistune-before-3.3.3-denial-of-service-via-balance… (third-party-advisory)

Frequently asked questions

What is CVE-2026-121438?: CVE-2026-121438 is a medium-severity vulnerability in Lepture Mistune, classified under Uncontrolled Resource Consumption. CVSS score: 5.3/10. Published 2026-11-03.
How severe is CVE-2026-121438?: Medium severity. CVSS v3 base score is 5.3 out of 10.