CVE-2026-33079

Name	CVE-2026-33079
Description	In versions 3.0.0a1 through 3.2.0 of Mistune, there is a ReDoS (Regular Expression Denial of Service) vulnerability in `LINK_TITLE_RE` that allows an attacker who can supply Markdown for parsing to cause denial of service. The regular expression used for parsing link titles contains overlapping alternatives that can trigger catastrophic backtracking. In both the double-quoted and single-quoted branches, a backslash followed by punctuation can be matched either as an escaped punctuation sequence or as two ordinary characters, creating an ambiguous pattern inside a repeated group. If an attacker supplies Markdown containing repeated ! sequences with no closing quote, the regex engine explores an exponential number of backtracking paths. This is reachable through normal Markdown parsing of inline links and block link reference definitions. A small crafted input can therefore cause significant CPU consumption and make applications using Mistune unresponsive.
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)
Debian Bugs	1135942

Vulnerable and fixed packages

The table below lists information on source packages.

Source Package	Release	Version	Status
mistune (PTS)	bullseye	0.8.4-4	fixed
	bookworm	2.0.4-1	fixed
	trixie	3.1.3-1	vulnerable
	forky, sid	3.1.4-1	vulnerable

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

Package	Type	Release	Fixed Version	Debian Bugs
mistune	source	bullseye	(not affected)
mistune	source	bookworm	(not affected)
mistune	source	(unstable)	(unfixed)	1135942

Notes

[trixie] - mistune <postponed> (Minor issue, revisit when fixed upstream)
[bookworm] - mistune <not-affected> (Vulnerable code not present)
[bullseye] - mistune <not-affected> (Vulnerable code not present)
https://github.com/lepture/mistune/security/advisories/GHSA-8mp2-v27r-99xp