CVE-2026-73321
XenForo < 2.3.13 Uncontrolled Recursion DoS via BBCode Parser
HIGH 7.1EPSS 0.6%
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
- XenForo
- CVSS v4.0 VULNCHECK
- 7.1 HIGH
CVSS:4.0/AV:N/AC:L/AT:N/PR:L/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X - CVSS v3.1 VULNCHECK
- 6.5 MEDIUM
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H - EPSS
- 0.61% chance of exploitation in the next 30 days, 48th percentile
- Published
- 2026-09-08
- Updated
- 2026-10-01
Proof-of-concept exploits (1)
- BomboBombone/CVE-2026-733210★ · 2026-09-08
Security advisories
- https://bombobombone.github.io/posts/cve-2026-73321
- https://www.vulncheck.com/advisories/xenforo-uncontrolled-recursion-dos-via-bbcode-parser
- https://xenforo.com/community/threads/security-fixes-released-for-all-xenforo-and-media-g…
- https://xenforo.com/community/threads/xenforo-2-3-13-and-add-ons-released-includes-securi…