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.
Scoring
- Severity
- CRITICAL
- CVSS base score
- 9.1
- CVSS vector
- 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
- EPSS probability
- 0.47%
- CWE
- CWE-697
- Published
- 2026-09-08
- Last modified
- 2026-09-09
Affected products
- XenForo XenForo
Weakness type
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