CVE-2026-73309 XenForo < 2.3.13 Authentication Bypass via OAuth2 Token Endpoint
Unauthenticated attackers can bypass OAuth2 validation in XenForo to obtain valid tokens and potentially take over accounts.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- None listed
What is CVE-2026-73309?
CVE-2026-73309 is a critical authentication bypass vulnerability in XenForo, published on 8 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official 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.
Weakness: CWE-697
Which versions are affected by CVE-2026-73309?
It affects 1 product.
Products include Xenforo Xenforo.
How do I fix CVE-2026-73309?
No fix has been published for CVE-2026-73309 yet. Limit exposure of the affected software until one is available.
Upgrade, and rotate credentials and sessions that may have been exposed. More about Authentication bypass.
Is CVE-2026-73309 being exploited?
It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-73309 exists.
Public exploit code: 1 proofs of concept on GitHub.
Other vulnerabilities in XenForo
Sources
Also known as GHSA-pr5w-fh2v-7c9q.