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

Vulnerability summary

CVE-2026-33079 is a high (CVSS 8.7) vulnerability in lepture mistune. EPSS estimates a 0.59% chance of exploitation in the next 30 days, higher than 48.3% of scored vulnerabilities.

Timeline

1. 06-05-2026
   PublishedVulnerability first disclosed.
2. 28-08-2026
   Last modifiedVulnerability information updated.

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.

• CWE-1333Inefficient Regular Expression ComplexityThe product uses a regular expression with a worst-case computational complexity that is inefficient and possibly exponential.