CVE-2026-73309

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.

• CWE-697

• https://bombobombone.github.io/posts/cve-2026-73309/Exploit, Third Party Advisory
• https://github.com/BomboBombone/CVE-2026-73309Third Party Advisory
• https://www.vulncheck.com/advisories/xenforo-authentication-bypass-via-oauth2-token-endpointThird Party Advisory
• https://xenforo.com/community/threads/security-fixes-released-for-all-xenforo-and-media-gallery-versions-2-2-0-2-3-12.239856/Release Notes
• https://xenforo.com/community/threads/xenforo-2-3-13-and-add-ons-released-includes-security-fixes.239857/Release Notes

Published

Sep 8, 2026

Last Modified

Sep 11, 2026

Status

Analyzed

• How Strix found a critical auth bypass in etcdStrix autonomously discovered a critical authentication bypass in etcd, later designated CVE-2026-33413.
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