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Advisory record · GHSA-ffq3-xpv3-j92q

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

CVSS 7.5 HighEPSS 0.0065 · 51st percentile

Published by the GitHub Advisory Database at 20 Jul 2026, 21:24 UTC. agentprime has not analysed this yet, so whether it affects any app is not known.

PackageEcosystemAffectedFixed in
mistunePyPI<3.3.03.3.0
Identifiers
GHSA-ffq3-xpv3-j92q · CVE-2026-59928 · PYSEC-2026-2216
Weakness (CWE)
CWE-407 · CWE-1333
Tags
Published
20 Jul 2026
Updated
20 Jul 2026

Machine-written summary

Summary written by Claude Haiku from the source advisory. Not reviewed by a person.

Mistune versions before 3.3.0 take quadratic time to parse a markdown document containing a long run of reference-link definitions. An attacker who can get the application to render attacker-supplied markdown can cause high CPU use with a modest input, and the cost grows with the square of the number of definitions. Upgrade mistune to 3.3.0 or later. The source gives few details on workarounds.

Written 5 Nov 2026.

Not analysed yet

We haven’t published our analysis of this Advisory yet. Once a person has reviewed it, this page will say what’s vulnerable, what has to be true for your app to be affected, and how to check.

Severity and exploitation

ScoreRatingCVSSVectorSource
7.5 High 3.1 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H CNA (GitHub_M)
  • CISA KEV Not listed.
  • EPSS 0.0065, the 51st percentile, on 15 Nov 2026: FIRST’s estimate of the chance of exploitation in the next 30 days.

Timeline

  1. 31 Oct 2026, 14:42 UTC First seen by agentprime
  2. 31 Oct 2026, 14:42 UTC Confirmed by OSV

Sources

Facts on this page come from the GitHub Advisory Database (CC-BY 4.0), OSV and the other sources listed, as published. We don’t reproduce full advisory descriptions, which can contain exploit code.