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high advisory

XenForo OAuth2 Unauthorized Scope Request Vulnerability

XenForo before 2.3.5 allows OAuth2 client applications to request unauthorized scopes, potentially allowing client applications to gain access beyond their intended authorization level due to improper authorization checks.

XenForo, a popular forum software, has a security vulnerability (CVE-2025-71278) affecting versions prior to 2.3.5. Specifically, the vulnerability lies in the OAuth2 client application authorization process. OAuth2 clients can request scopes beyond those they are authorized to access. This vulnerability impacts any XenForo 2.3 installation utilizing OAuth2 clients prior to upgrading to version 2.3.5. Successful exploitation could allow malicious or compromised OAuth2 client applications to escalate privileges and access sensitive data or functionality within the XenForo forum.

Attack Chain

  1. Attacker registers a malicious OAuth2 client application within the vulnerable XenForo instance.
  2. The attacker crafts an OAuth2 authorization request, including scopes that the client should not be permitted to access according to XenForo’s intended authorization model.
  3. The vulnerable XenForo instance fails to properly validate the requested scopes against the client’s authorized permissions.
  4. The XenForo server grants access tokens with the requested, unauthorized scopes.
  5. The malicious OAuth2 client application uses the access token with the expanded privileges to interact with the XenForo API.
  6. The attacker performs actions they are not intended to be authorized for, such as accessing private user data, modifying forum settings, or performing administrative tasks depending on the scopes gained.

Impact

Successful exploitation of CVE-2025-71278 can lead to unauthorized data access, privilege escalation, and potential compromise of the XenForo forum. This can impact all users of the forum, leading to data breaches, defacement, or disruption of service. The severity depends on the unauthorized scopes obtained, but could range from accessing private messages to complete administrative control over the forum.

Recommendation

  • Upgrade XenForo installations to version 2.3.5 or later to remediate CVE-2025-71278 (reference: XenForo advisory in references).
  • Implement rate limiting on OAuth2 authorization requests to identify and mitigate potential abuse (reference: generic security best practice).

Detection coverage 2

Detect Suspicious OAuth2 Scope Request

medium

Detects OAuth2 authorization requests containing unusual or excessive scopes, potentially indicating an attempt to exploit CVE-2025-71278.

sigma tactics: initial_access techniques: T1588.004 sources: webserver, linux

Detect XenForo OAuth2 Admin Scope Request

high

Detects requests containing the string 'admin' in the OAuth2 scope, which may be indicative of privilege escalation attempts in XenForo.

sigma tactics: privilege_escalation techniques: T1068 sources: webserver, linux

Detection queries are kept inside the platform. Get full rules →

Indicators of compromise

1

email

1

url

TypeValue
urlhttps://nvd.nist.gov
email[email protected]