CVE-2026-86435: Thephpleague Commonmark

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

commonmark versions from 1.5.0 before 2.8.4 contain a denial of service vulnerability in the Footnote extension that fails to deduplicate footnote definitions. Attackers can craft documents with duplicate footnote definitions and references to create quadratic output expansion, consuming excessive memory and CPU to exhaust server resources.

Affected products

• Thephpleague Commonmark: from 1.5.0, before 2.8.4 (fixed in 2.8.4)

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