CVE-2026-86434: Inefficient Algorithmic Complexity in thephpleague commonmark
Description
Thephpleague commonmark versions 2.0.0 up to but not including 2.8.4 contain a denial of service vulnerability due to inefficient algorithmic complexity in the UniqueSlugNormalizer::normalize() function. This vulnerability occurs when certain extensions are registered, allowing an attacker to cause excessive CPU consumption by forcing many headings to collapse to the same base slug. The issue was patched in version 2.9.0.
CVSS v4.0
Score 8.7high
Affected software
thephpleague
commonmark
pkg:composer/league/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-2026-86434 describes a denial of service vulnerability in thephpleague commonmark library versions >=2.0.0 and <2.8.4. The vulnerability arises from the UniqueSlugNormalizer::normalize() method, which restarts its numeric suffix search from 1 on every slug collision, resulting in O(K^2) time complexity for K headings that share the same base slug. This inefficient algorithmic complexity can be triggered when the HeadingPermalinkExtension, FootnoteExtension, or TableOfContentsExtension is registered. An unauthenticated attacker can exploit this by crafting a Markdown document with many headings that collapse to the same slug (e.g., empty or identical headings), causing excessive CPU usage and denial of service.
Potential Impact
An unauthenticated attacker can cause a denial of service by forcing the commonmark processor to consume excessive CPU resources when processing Markdown documents with many headings that collapse to the same base slug. This can degrade or disrupt service availability for applications using affected versions of thephpleague commonmark with the vulnerable extensions enabled.
Mitigation Recommendations
A fix is available in version 2.9.0 of thephpleague commonmark. Users should upgrade to version 2.9.0 or later to remediate this vulnerability. No other mitigations are specified.
Technical Details
- Data Version
- 5.2
- Assigner Short Name
- VulnCheck
- Date Reserved
- 2026-09-07T12:34:31.457Z
- Cvss Version
- 4.0
- State
- PUBLISHED
Threat ID: 6a9eb72eacd9273b49a9cfee
Added to database: 09/07/2026, 13:07:58 UTC
Last enriched: 10/09/2026, 21:25:35 UTC
Last updated: 10/21/2026, 13:20:24 UTC
Views: 85
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