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Vulnerability record · CVE-2026-73309 · published 8 September 2026

CVE-2026-73309: Xenforo vulnerability

Xenforo · Xenforo

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.

9.1 CVSS 4.0 Critical EPSS 0.69% · top 49.4% CWE-697 · CWE-697
9.1CVSS 4.0 base score
0.69%EPSS exploitation probability, 30 days
NoNot in CISA KEV
1Affected product versions listed by NVD
5References, 1 tagged exploit
11 Sep 2026Last modified by NVD

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.

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:

Affected products

1 vulnerable configurations from NVD's CPE data, grouped by vendor and product.

References

LinkTags
https://bombobombone.github.io/posts/cve-2026-73309/ ExploitThird Party Advisory
https://github.com/BomboBombone/CVE-2026-73309 Third Party Advisory
https://www.vulncheck.com/advisories/xenforo-authentication-bypass-via-oauth2-token-endpoint Third Party Advisory
https://xenforo.com/community/threads/security-fixes-released-for-all-xenforo-and-media-gallery-versions-2-2-0-2-3-12.23 Release Notes
https://xenforo.com/community/threads/xenforo-2-3-13-and-add-ons-released-includes-security-fixes.239857/ Release Notes

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Related vulnerabilities

Same products first, then exploited flaws of the same weakness class.

9.3CVE-2025-71279Xenforo improper authentication vulnerabilityXenForo before 2.3.7 contains a security issue affecting Passkeys that have been added to user accounts. An attacker may be able to compromise the se…EPSS 0.45%9.1CVE-2026-73311Xenforo authentication bypass by capture-replay vulnerabilityXenForo before 2.3.13 contains an OAuth2 authorization code reuse vulnerability that allows attackers to obtain unauthorized token pairs by submittin…EPSS 0.50%9.1CVE-2026-73312Xenforo authentication bypass by capture-replay vulnerabilityXenForo before 2.3.13 contains a refresh token replay vulnerability that allows attackers to reuse a refresh token multiple times by exploiting the f…EPSS 0.47%8.8CVE-2024-38457Xenforo cross-site request forgery vulnerabilityXenforo before 2.2.16 allows CSRF.EPSS 7.4%8.8CVE-2024-38458Xenforo code injection vulnerabilityXenforo before 2.2.16 allows code injection.EPSS 0.89%8.7CVE-2026-73314Xenforo vulnerabilityXenForo before 2.3.13 contains a signature verification logic error in the PayPal REST webhook handler that allows unauthenticated attackers to bypas…EPSS 0.65%8.7CVE-2026-73316Xenforo insufficient verification of data authenticity vulnerabilityXenForo before 2.3.13 contains a payment replay vulnerability in the PayPal REST payment provider that allows attackers to process the same webhook p…EPSS 0.27%8.7CVE-2025-71278Xenforo incorrect authorization vulnerabilityXenForo before 2.3.5 allows OAuth2 client applications to request unauthorized scopes. This affects any customer using OAuth2 clients on any version …EPSS 0.27%

Source: NIST National Vulnerability Database (record CVE-2026-73309), CISA KEV, FIRST EPSS (scores of 2026-09-26). This page is refreshed as NVD updates the record.