CVE-2026-59928Mistune block_parser: quadratic-time parsing on long lists of repeated reference-link definitions

Description

Mistune is a Python Markdown parser with renderers and plugins. Prior to 3.3.0, a Markdown document containing many repeated or distinct reference-link definitions causes quadratic work in src/mistune/block_parser.py and the ref_links environment dictionary handling, allowing denial of service through CPU exhaustion. This issue is fixed in version 3.3.0.

Data: CVE.org, NVD, CISA KEV, FIRST EPSS, Ubuntu. Published as the sources state it, in English.

Affected products and versions

References and patches

• https://github.com/lepture/mistune/security/advisories/GHSA-ffq3-xpv3-j92q ExploitThird Party Advisory
• https://github.com/lepture/mistune/commit/2b04d7ba341c16ac78fe82d3076bdd5c3de87c69 Patch
• https://github.com/lepture/mistune/releases/tag/v3.3.0 Release Notes