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CVE-2026-121437 Details

ANALYZED

This CVE record has been analyzed and enriched by NVDAPI.com as an independent party.

Description

Mistune from 3.3.0 before 3.3.4 contains a denial of service vulnerability that allows remote attackers to exhaust CPU through quadratic-time emphasis delimiter processing. Attackers can submit small markdown documents with repeated patterns like "*a_b" to trigger an O(n²) index-fixup loop, stalling workers for seconds.

Metrics

CVSS 3.x Severity and Vector Strings:

VolerionCVSS-B:5.3 MEDIUMVector:CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L

CNA: [email protected]CVSS-B:7.5 HIGHVector:CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

References to Advisories, Solutions, and Tools

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URL	Source(s)	Tag(s)
https://github.com/lepture/mistune/security/advisories/GHSA-qp42-r9j6-83g9	CISA-ADP	AdvisoryRemedy
https://github.com/lepture/mistune/commit/b4dd3284e52ec741ed9a227db29ade9017828e90	[email protected]	Source Code
https://github.com/lepture/mistune/security/advisories/GHSA-qp42-r9j6-83g9	[email protected]	AdvisoryRemedy
https://www.vulncheck.com/advisories/mistune-3.3.0-before-3.3.4-quadratic-time-dos-via-process-emphasis-delimiters	[email protected]	AdvisoryRemedy

Weakness Enumeration

CWE-ID	CWE Name	Source
CWE-407	Inefficient Algorithmic Complexity	[email protected]

Affected Products

Product	Versions
Mistune	>= 3.3.0, < 3.3.4 (semver)
CPE cpe:2.3:a:mistune_project:mistune:::::::: Remediation Upgrade: 3.3.4moderate effort

Change History

2 change records found show changes

Date	Action	Recorded By
Nov 3, 2026	CVE Modified	CISA-ADP
Nov 3, 2026	New CVE Received	[email protected]