# CVE-2026-73321

## Summary

- **CVE ID:** CVE-2026-73321
- **Severity:** HIGH
- **CVSS Score:** 7.1 (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)
- **CWE:** CWE-674
- **Published:** Sep 8, 2026
- **Last Modified:** Sep 9, 2026

## 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

- XenForo — XenForo (0)

## References

- [CNA](https://xenforo.com/community/threads/security-fixes-released-for-all-xenforo-and-media-gallery-versions-2-2-0-2-3-12.239856/)
- [CNA](https://xenforo.com/community/threads/xenforo-2-3-13-and-add-ons-released-includes-security-fixes.239857/)
- [CNA](https://www.vulncheck.com/advisories/xenforo-uncontrolled-recursion-dos-via-bbcode-parser)
- [CNA](https://bombobombone.github.io/posts/cve-2026-73321/)
- [CNA](https://github.com/BomboBombone/CVE-2026-73321)

## Exploitation Prediction (EPSS)

- **EPSS Score:** 0.38%
- **EPSS Percentile:** 31.4

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_Exported from OnDuty AI Vulnerability Intelligence on 2026-09-10._