CVE-2026-73321 [MEDIUM] – XenForo before 2.3.13 contains an uncontrolled recursion vulnerability in the BBCode parse

XenForo before 2.3.13 contains an uncontrolled recursion vulnerability in the BBCode parser that allows authenticated attackers to cause persistent denial of service by submitting a post with deeply nested BBCode tags. Attackers can craft a single malicious post with sufficient nesting depth to exceed PHP’s stack limit, causing fatal errors that repeatedly terminate PHP-FPM workers for all visitors rendering the affected thread.