CVE-2026-73311 in XenForo
Summary
by MITRE • 09/08/2026
XenForo before 2.3.13 contains an OAuth2 authorization code reuse vulnerability that allows attackers to obtain unauthorized token pairs by submitting a previously used authorization code. Attackers can exploit the failure to invalidate or mark authorization codes as consumed after initial token issuance to receive an independent token pair for the same user and scopes, bypassing the single-use guarantee of the OAuth2 authorization code flow.
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Analysis
by VulDB Data Team • 09/08/2026
The vulnerability identified in XenForo versions prior to 2.3.13 represents a critical failure in the implementation of the OAuth 2.0 Authorization Code Grant Flow, specifically concerning the lifecycle management of authorization codes. In standard OAuth 2.0 protocols as defined by RFC 6749, an authorization code is designed to be single-use only. Once a client exchanges this temporary code for access and refresh tokens at the token endpoint, the server must immediately invalidate that specific code to prevent its reuse. The flaw in XenForo stems from the application's failure to enforce this mandatory invalidation step. Consequently, if an attacker intercepts or obtains a valid authorization code before it is exchanged by the legitimate client, they can submit that same code multiple times to the token endpoint. Each submission results in the issuance of a new, independent pair of access and refresh tokens for the targeted user account within the specified scopes.
This technical flaw directly undermines the security guarantees provided by the OAuth 2.0 framework, which relies on the ephemeral nature of authorization codes to mitigate risks associated with code interception or leakage during transmission between the browser-based client and the application server. By allowing multiple exchanges of a single code, XenForo effectively transforms what should be a one-time credential into a persistent secret that can be exploited repeatedly without requiring additional authentication factors such as passwords or multi-factor authentication tokens. This behavior creates a significant opportunity for unauthorized access, particularly in scenarios where an attacker might have partial visibility into the authorization flow, such as through cross-site scripting attacks targeting the redirect URI parameters or via man-in-the-middle attacks on unsecured connections during the initial code exchange phase.
The operational impact of this vulnerability is severe, as it allows attackers to gain persistent unauthorized access to user accounts without needing to compromise long-term credentials like passwords. Once an attacker obtains a valid token pair through code reuse, they can utilize these tokens to interact with the XenForo API and application features on behalf of the victim. This includes accessing private messages, viewing personal profile information, modifying account settings, or performing actions that require specific user permissions depending on the scopes granted during authorization. The persistence provided by refresh tokens further exacerbates this risk, as it allows attackers to maintain access over extended periods even if short-lived access tokens expire, thereby facilitating long-term reconnaissance and potential data exfiltration or defacement of community content.
From a classification perspective, this vulnerability aligns with CWE-294, which describes the acceptance of a duplicate certificate for authentication, although more accurately it maps to CWE-613: Insufficient Session Expiration. The failure to invalidate the authorization code after its first use violates fundamental principles of secure session management and token lifecycle control. In terms of offensive security tactics, this exploitation technique corresponds to MITRE ATT&CK Tactic TA0004 (Privilege Escalation) under techniques such as T1528: Steal Application Access Token or more broadly within the initial access phase if used in conjunction with phishing or XSS attacks that facilitate code interception. It also relates to CWE-347, Improper Verification of Cryptographic Signature, in a broader sense regarding the verification of token validity and state integrity during exchange processes.
To mitigate this vulnerability, administrators must upgrade XenForo to version 2.3.13 or later, where the developers have implemented proper server-side tracking to ensure that authorization codes are marked as consumed immediately upon successful token issuance. For environments unable to patch immediately due to operational constraints, implementing strict monitoring of OAuth token endpoint requests can help detect anomalous patterns indicative of code reuse attempts, such as multiple rapid exchanges using identical authorization codes for a single user session. Additionally, enforcing the use of PKCE (Proof Key for Code Exchange) adds an additional layer of security by binding the authorization request to the client application through a secret verifier, making it significantly more difficult for attackers who intercept the authorization code to successfully exchange it unless they also possess the corresponding challenge and verifier generated during the initial flow.