CVE-2026-121438

Name	CVE-2026-121438
Description	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.
Source	CVE (at NVD; CERT, ENISA, LWN, oss-sec, fulldisc, Debian ELTS, Red Hat, Ubuntu, Gentoo, SUSE bugzilla/CVE, GitHub advisories/code/issues, web search, more)

Vulnerable and fixed packages

The table below lists information on source packages.

Source Package	Release	Version	Status
mistune (PTS)	bookworm	2.0.4-1	vulnerable
	trixie	3.1.3-1	vulnerable
	forky	3.1.4-1	vulnerable
	sid	3.4.0-1	fixed

The information below is based on the following data on fixed versions.

Package	Type	Release	Fixed Version	Urgency	Origin	Debian Bugs
mistune	source	(unstable)	3.3.4-1

Notes

[trixie] - mistune <no-dsa> (Minor issue)
https://github.com/lepture/mistune/security/advisories/GHSA-47p6-w2rc-5qf7
https://github.com/lepture/mistune/commit/fe02f40faf091f86da9ed62c865f31e11da111d7 (v3.3.3)