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

Attack Vector Network

Attack Complexity Low

Privileges Required None

Attack Requirements None

User Interaction None

Vulnerable System Confidentiality Impact None

Vulnerable System Integrity Impact None

Vulnerable System Availability Impact High

Subsequent System Confidentiality Impact None

Subsequent System Integrity Impact None

Subsequent System Availability Impact None

Attack Vector Network

Attack Complexity Low

Privileges Required None

Scope Unchanged

Confidentiality Impact None

Integrity Impact None

Availability Impact High

User Interaction None

No CVSS v3.0

No CVSS v2

This CVE is not in the KEV list.

No EPSS score available.

Key SSVC decision points have not yet been added.

Affected Vendors & Products

Vendors Products
Mistune Project Subscribe
Mistune Subscribe

No data.

No data.

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Advisories

No advisories yet.

Fixes

Solution

No solution given by the vendor.

Workaround

No workaround given by the vendor.

References
Link Providers
https://github.com/lepture/mistune/commit/b4dd3284e52ec741ed9a227db29ade9017828e90 cve-icon cve-icon
https://github.com/lepture/mistune/security/advisories/GHSA-qp42-r9j6-83g9 cve-icon cve-icon
https://www.vulncheck.com/advisories/mistune-3.3.0-before-3.3.4-quadratic-time-dos-via-process-emphasis-delimiters cve-icon cve-icon
History

Tue, 03 Nov 2026 14:45:00 +0000

Type Values Removed Values Added
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.
Title Mistune 3.3.0 before 3.3.4 Quadratic-Time DoS via _process_emphasis_delimiters
First Time appeared Mistune Project
Mistune Project mistune
Weaknesses CWE-407
CPEs cpe:2.3:a:mistune_project:mistune:*:*:*:*:*:*:*:*
Vendors & Products Mistune Project
Mistune Project mistune
References
Metrics cvssV3_1

{'score': 7.5, 'vector': 'CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H'}

cvssV4_0

{'score': 8.7, 'vector': 'CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N'}

Projects

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cve-icon MITRE

Status: PUBLISHED

Assigner: VulnCheck

Published:

Updated: 2026-11-03T14:28:29.178Z

Reserved: 2026-11-03T12:32:14.752Z

Link: CVE-2026-121437

cve-icon Vulnrichment

No data.

cve-icon NVD

Status : Deferred

Published: 2026-11-03T15:17:35.147

Modified: 2026-11-03T15:17:35.280

Link: CVE-2026-121437

cve-icon Redhat

No data.

cve-icon OpenCVE Enrichment

Updated: 2026-11-03T16:15:13Z

Weaknesses