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

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Summary

CVECVE-2026-59928
StatePUBLISHED
AssignerGitHub_M
Source PriorityCVE Program / NVD first with legacy fallback
Published2026-07-08 17:17:28 UTC
Updated2026-07-09 19:29:34 UTC
DescriptionMistune 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.

Risk And Classification

Primary CVSS: v3.1 7.5 HIGH from [email protected]

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

EPSS: 0.003460000 probability, percentile 0.266430000 (date 2026-07-09)

Problem Types: CWE-407 | CWE-1333 | CWE-407 CWE-407: Inefficient Algorithmic Complexity | CWE-1333 CWE-1333: Inefficient Regular Expression Complexity

VersionSourceTypeScoreSeverityVector
3.1[email protected]Secondary7.5HIGHCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
3.1CNADECLARED7.5HIGHCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

CVSS v3.1 Breakdown

Attack Vector
Network
Attack Complexity
Low
Privileges Required
None
User Interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
None
Availability
High

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

NVD Known Affected Configurations (CPE 2.3)

TypeVendorProductVersionUpdateEditionLanguage
Application Mistune Project Mistune All All All All

Vendor Declared Affected Products

SourceVendorProductVersionPlatforms
CNA Lepture Mistune affected < 3.3.0 Not specified

References

ReferenceSourceLinkTags
github.com/lepture/mistune/commit/2b04d7ba341c16ac78fe82d3076bdd5c3de87c69 [email protected] github.com Patch
github.com/lepture/mistune/releases/tag/v3.3.0 [email protected] github.com Release Notes
github.com/lepture/mistune/security/advisories/GHSA-ffq3-xpv3-j92q 134c704f-9b21-4f2e-91b3-4a467353bcc0 github.com Exploit, Third Party Advisory
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