Vulnerability · Published Sep 8, 2026 · Assigned by VulnCheck
CVE-2026-73321XenForo < 2.3.13 Uncontrolled Recursion DoS via BBCode Parser
Medium 6.5
In plain English
A medium-severity security flaw in XenForo from XenForo. Proof-of-concept exploit code is public. The chance it is exploited in the next 30 days is estimated at 0.6% (higher than 50% of all known vulnerabilities). Ready-made attack tooling exists: public proof-of-concept code.
Official description
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.
Affected products (as reported)
Weakness: CWE-674 · Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
References
- Vendor Advisory vendor-advisory, patch
- Release Notes release-notes
- Researcher Advisory technical-description
- Researcher PoC exploit
- https://www.vulncheck.com/advisories/xenforo-uncontrolled-recursion-dos-via-bbcode-parser third-party-advisory