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Vulnerability · Published May 6, 2026 · Assigned by GitHub_M

CVE-2026-33079Mistune ReDoS in LINK_TITLE_RE allows denial of service with crafted Markdown titles

High 7.5

In plain English

A high-severity regular-expression denial of service in mistune from lepture. Proof-of-concept exploit code is public. The chance it is exploited in the next 30 days is estimated at 0.6% (higher than 48% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.

Severity (CVSS 3.1)7.5High
Exploit odds (30 days)0.6%top 51.5% of all CVEs
Exploited in the wildNo reportsCISA SSVC: poc
Attack toolingNoneno public tooling known

Official 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.

Affected packages & fixed versions

EcosystemPackageFixed inAdvisory
Debian:13mistuneno fix listedDEBIAN-CVE-2026-33079
Debian:14mistuneno fix listedDEBIAN-CVE-2026-33079
PyPImistune3.2.1GHSA-8mp2-v27r-99xp
PyPImistune3.2.1PYSEC-2026-2651
Ubuntu:22.04:LTSmistuneno fix listedUBUNTU-CVE-2026-33079
Ubuntu:24.04:LTSmistuneno fix listedUBUNTU-CVE-2026-33079
Ubuntu:25.10mistuneno fix listedUBUNTU-CVE-2026-33079
Ubuntu:26.04:LTSmistuneno fix listedUBUNTU-CVE-2026-33079
Ubuntu:26.10mistuneno fix listedUBUNTU-CVE-2026-33079

Affected products (as reported)

VendorProductVersions
lepture mistune >=3.0.0a1, <= 3.2.0

Weakness: CWE-1333 · Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

References