# CVE-2026-73309: XenForo authentication bypass in OAuth2 token endpoint

Source: https://junglewise.ai/threats/cve-2026-73309-xenforo-authentication-bypass-in-oauth2-token-endpoint  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-09-08. Updated: 2026-09-26  
License: CC BY 4.0 with a link back

CVE-2026-73309 · Severity: high · CVSS 7.4 · Published 2026-09-08

XenForo is a popular community forum platform that includes OAuth2 authentication for third-party applications. A vulnerability in the OAuth2 token endpoint allows attackers who obtain a valid authorization code to bypass PKCE (Proof Key for Code Exchange) verification and client secret validation by submitting empty strings. This enables attackers to obtain forged access tokens without proving their identity, potentially allowing them to impersonate legitimate users and access their authorized data.

## Quick answers

**Is CVE-2026-73309 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-73309 being exploited in the wild.

**How severe is CVE-2026-73309?** High severity, with a CVSS score of 7.4, published 8 September 2026.

**Which products and versions are affected?** XenForo XenForo before 2.3.13.

**Is there a fix for CVE-2026-73309?** A patch was published on 2026-09-08: Fixed in XenForo 2.3.13 Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-73309 attacked?** It is attacked over the network, in a web application.

**Am I affected by CVE-2026-73309?** Check whether anything you run is named in the advisory. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## Technical details

The vulnerability is an authentication bypass in XenForo's OAuth2 token endpoint caused by improper input validation. The token endpoint checks whether client_secret and code_verifier parameters exist but fails to validate they are non-empty before performing cryptographic verification. PHP's truthy evaluation treats empty strings as false, causing the verification logic guarded by if ($input['client_secret']) and if ($input['code_verifier']) conditionals to be skipped entirely. An attacker with a valid authorization code can exchange it for a token pair without providing the PKCE verifier or client secret. The vulnerability affects both the authorization-code and refresh-token grant types. XenForo 2.3.13 fixes the issue by rejecting empty credentials before processing grants.

## Affected products

- XenForo XenForo before 2.3.13

## Timeline

- 2026-09-08: disclosed
- 2026-09-08: patched: Fixed in XenForo 2.3.13

## References

- [https://bombobombone.github.io/posts/cve-2026-73309/](https://bombobombone.github.io/posts/cve-2026-73309/)
- [https://github.com/BomboBombone/CVE-2026-73309](https://github.com/BomboBombone/CVE-2026-73309)