High severity7.4NVD Advisory· Published Sep 8, 2026· Updated Sep 11, 2026
CVE-2026-73309
CVE-2026-73309
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.
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Affected products
2Patches
Vulnerability mechanics
References
4- bombobombone.github.io/posts/cve-2026-73309/nvdExploitThird Party Advisory
- www.vulncheck.com/advisories/xenforo-authentication-bypass-via-oauth2-token-endpointnvdThird Party Advisory
- xenforo.com/community/threads/security-fixes-released-for-all-xenforo-and-media-gallery-versions-2-2-0-2-3-12.239856/nvdRelease Notes
- xenforo.com/community/threads/xenforo-2-3-13-and-add-ons-released-includes-security-fixes.239857/nvdRelease Notes
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