CVE-2026-86429: Inefficient Algorithmic Complexity in thephpleague commonmark
Description
Thephpleague/commonmark library versions 1.5.0 up to but not including 2.9.1 contain a vulnerability causing quadratic parsing complexity in the SmartPunctExtension and AttributesExtension. These extensions are not enabled by default. An unauthenticated attacker can exploit this by submitting crafted Markdown documents to cause excessive CPU usage and denial of service. The issue is fixed in version 2.9.1.
CVSS v4.0
Score 8.7high
Affected software
thephpleague
commonmark
pkg:composer/thephpleague/commonmarkRun on your own infrastructure? Check whether these packages are installed with threat-finder — our free open-source scanner.
AI-Powered Analysis
Machine-generated threat intelligence
Technical Analysis
CVE-2026-86429 describes an inefficient algorithmic complexity vulnerability in thephpleague/commonmark library versions >=1.5.0 and <2.9.1. Specifically, the SmartPunctExtension and AttributesExtension exhibit quadratic parsing complexity when explicitly registered on the Environment. This allows an unauthenticated attacker to submit specially crafted Markdown input that triggers disproportionate CPU consumption, resulting in denial of service. These extensions are not enabled by default and are excluded from standard CommonMark and GitHub-Flavored Markdown converters. The vulnerability is resolved in version 2.9.1.
Potential Impact
An unauthenticated attacker can cause a denial of service by submitting small, specially crafted Markdown documents that exploit quadratic parsing complexity in the affected extensions. This leads to excessive CPU consumption, potentially impacting availability of services using the vulnerable library with these extensions enabled.
Mitigation Recommendations
Upgrade thephpleague/commonmark to version 2.9.1 or later, where this vulnerability is fixed. If upgrading is not immediately possible, avoid enabling the SmartPunctExtension and AttributesExtension, as they are not enabled by default and are the source of the vulnerability.
Technical Details
- Data Version
- 5.2
- Assigner Short Name
- VulnCheck
- Date Reserved
- 2026-09-07T12:33:13.368Z
- Cvss Version
- 4.0
- State
- PUBLISHED
Threat ID: 6a9eb72cacd9273b49a9cf73
Added to database: 09/07/2026, 13:07:56 UTC
Last enriched: 10/09/2026, 21:25:09 UTC
Last updated: 10/22/2026, 01:15:26 UTC
Views: 72
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