CVE-2026-86434: Inefficient Algorithmic Complexity in thephpleague commonmark
Description
league/commonmark versions >= 2.0.0 and < 2.8.4 (patched in 2.9.0) contain a denial of service vulnerability in UniqueSlugNormalizer::normalize(), which restarts its numeric-suffix search from 1 on every slug collision, resulting in O(K^2) time complexity for K headings that collapse to the same base slug. The vulnerable path is reached when HeadingPermalinkExtension, FootnoteExtension, or TableOfContentsExtension is registered. An unauthenticated attacker can force many headings onto a single base slug (e.g., via empty ATX headings, identical heading text, or punctuation-only headings) in a small Markdown document, consuming excessive CPU and denying service.
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 affects thephpleague commonmark library versions >=2.0.0 and <2.8.4. The UniqueSlugNormalizer::normalize() method 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 vulnerable code path is triggered when HeadingPermalinkExtension, FootnoteExtension, or TableOfContentsExtension is enabled. 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 parser to consume excessive CPU resources when processing Markdown documents with many headings collapsing to the same slug. This can disrupt availability of services using the affected library versions with the specified 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.
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: 09/19/2026, 22:18:57 UTC
Last updated: 10/07/2026, 18:48:24 UTC
Views: 72
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