CVE-2026-86430

league/commonmark versions before 2.9.1 contain multiple denial of service vulnerabilities in fenced code block detection, reference link label lookup, and emphasis delimiter processing that perform super-linear work on crafted input. Attackers can submit specially crafted Markdown with long backtick runs, nested brackets, or delimiter sequences to consume disproportionate CPU time and prevent legitimate requests from completing.

• https://github.com/thephpleague/commonmark/security/advisories/GHSA-j8pm-gj4c-rq4x
• https://www.vulncheck.com/advisories/league-commonmark-before-2.9.1-denial-of-service-via-parsing