CVE-2026-33079: Mistune has a ReDoS in LINK_TITLE_RE that allows denial of service via crafted Markdown input

May 6, 2026

A ReDoS (Regular Expression Denial of Service) vulnerability in LINK_TITLE_RE allows an attacker who can supply Markdown for parsing to cause denial of service. A crafted 58-byte Markdown document blocks the parser for approximately 6 seconds (measured on Apple M2, Python 3.14.3), with exponential growth per additional byte pair.

References

github.com/advisories/GHSA-8mp2-v27r-99xp
github.com/lepture/mistune/blob/df23edd60b43b639d2e6760ef9dd3d618aa11c21/src/mistune/helpers.py
github.com/lepture/mistune/security/advisories/GHSA-8mp2-v27r-99xp
nvd.nist.gov/vuln/detail/CVE-2026-33079

Code Behaviors & Features

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