CVE-2026-86434: Thephpleague Commonmark

High severity, CVSS 7.5. EPSS: 0.5% chance of exploitation in the next 30 days.

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.

Affected products

• Thephpleague Commonmark: from 2.0.0, before 2.8.4 (fixed in 2.8.4)

Published 2026-09-07. Last modified 2026-10-08.