by Kastro
Paper/Archive/CVE-2026-73309

CVE-2026-73309

XenForo XenForo — XenForo < 2.3.13 Authentication Bypass via OAuth2 Token Endpoint

EPSS 0.7%public exploitCVSS 9.1
critical.is priorityof 100
48
CVSS 9.1 (CNA)9.1 / 10 × 3027.3Not in CISA KEV+300.0EPSS percentile 52.5.525 × 2010.5Exploit: SSVC poc+1010.0SSVC exploitation: poc+100.0
Description · from the CVE record

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

ProductAffectedFixed inSource
XenForo< 2.3.132.3.13CNA

Scores by source

VulnCheck (CNA)9.1CVSS:4.0/AV:N/AC:H/AT:N/PR:N/UI:N/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N
VulnCheck (CNA)7.4CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N
CISA ADPSSVCExploitation: poc · Automatable: no · Technical impact: total

References

patchxenforo.com/community/threads/security-fixes-released-for-all-xenforo-and-media-gallery-versions-2-2-0-2-3-12.239856refxenforo.com/community/threads/xenforo-2-3-13-and-add-ons-released-includes-security-fixes.239857writeupbombobombone.github.io/posts/cve-2026-73309exploitgithub.com/BomboBombone/CVE-2026-73309writeupvulncheck.com/advisories/xenforo-authentication-bypass-via-oauth2-token-endpoint