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
References5
- bombobombone.github.io/posts/cve-2026-73309/ ExploitThird Party Advisory
- github.com/BomboBombone/CVE-2026-73309 Third Party Advisory
- www.vulncheck.com/advisories/xenforo-authentication-bypass-via-… Third Party Advisory
- xenforo.com/community/threads/security-fixes-released-for-all-x… Release Notes
- xenforo.com/community/threads/xenforo-2-3-13-and-add-ons-releas… Release Notes
Metrics
- CVSS 3.1
-
9.1
CRITICAL
Score source: NVD
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N - CVSS 4.0
-
9.1
CRITICAL
Score source: CNA
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/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X - EPSS — exploit probability
- 1% percentile: 53 · as of 3 November 2026
- CWE
- CWE-697
Could this be exploited in your environment?
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