# CVE-2026-73309

## Summary

- **CVE ID:** CVE-2026-73309
- **Severity:** CRITICAL
- **CVSS Score:** 9.1 (CVSS:4.0/AV:N/AC:H/AT:N/PR:N/UI:N/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N)
- **CWE:** CWE-697
- **Published:** Sep 8, 2026
- **Last Modified:** Sep 9, 2026

## Description

XenForo before 2.3.13 contains an authentication bypass vulnerability in the OAuth2 token endpoint that allows unauthenticated attackers to obtain valid token pairs by submitting empty values for client_secret and code_verifier parameters. Attackers can exploit PHP truthy evaluation logic, which treats empty strings as false and skips client secret validation and PKCE code verifier validation, to exchange a valid authorization code for a token pair without proving client identity or holding the PKCE commitment.

## 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-authentication-bypass-via-oauth2-token-endpoint)
- [CNA](https://bombobombone.github.io/posts/cve-2026-73309/)
- [CNA](https://github.com/BomboBombone/CVE-2026-73309)

## Exploitation Prediction (EPSS)

- **EPSS Score:** 0.47%
- **EPSS Percentile:** 39.2

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