Privilege Escalation in MyBB via Usergroup Assignment

Executive Summary

CVE-2026-58054 is a privilege escalation vulnerability in MyBB version 1.8.40 that allows a user with limited Admin Control Panel (ACP) permissions, specifically delegated user-management rights, to assign the Administrators group to any user account.

The root cause is that the user module does not restrict which usergroup a limited ACP user may assign, and the verify_usergroup() function in the data handler always returns true without proper authorization checks.

This means an attacker with only user-management permission can create or edit users and assign them full Administrator privileges, effectively escalating their own permissions to full administrator level.

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Impact Analysis

This vulnerability can lead to unauthorized privilege escalation within the MyBB forum software.

• An attacker with limited Admin CP user-management permissions can gain full Administrator access.
• The attacker can modify board settings, manage all users, and alter forum data.
• This compromises the integrity and security of the forum, potentially leading to data breaches or unauthorized control.

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Detection Guidance

This vulnerability can be detected by checking if any user accounts have been assigned to the Administrators group (gid 4) by a limited Admin Control Panel user who should not have such privileges.

One approach is to audit user group assignments in the MyBB database, especially focusing on recent changes made by users with limited ACP permissions.

Since the issue involves the user module forwarding the usergroup field without proper authorization, monitoring logs for user creation or modification events where the Administrators group is assigned can help detect exploitation.

Specific commands depend on your environment, but for example, if you have direct database access, you can run SQL queries to identify users in the Administrators group and check who created or modified them.

• Example SQL query to list users in the Administrators group (gid=4): SELECT uid, username, usergroup, additionalgroups FROM mybb_users WHERE usergroup = 4 OR FIND_IN_SET(4, additionalgroups);
• Check logs or audit trails for user creation or modification actions performed by limited ACP users assigning gid 4.

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Mitigation Strategies

Immediate mitigation involves restricting the ability to assign the Administrators group (gid 4) only to super administrators or users with explicit high-trust permissions.

Since the vulnerability arises from the verify_usergroup() function unconditionally returning true, applying a patch or update that enforces stricter group assignment validation is critical.

If a patch is not yet available, temporarily revoke or limit the delegated user-management permissions from limited ACP users to prevent them from assigning the Administrators group.

Additionally, audit current user accounts for unauthorized Administrator group assignments and revert any suspicious changes.

Monitor your system closely for any signs of privilege escalation attempts and consider disabling or restricting the user management module for limited ACP users until a fix is applied.

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Compliance Impact

This vulnerability allows a user with limited Admin Control Panel permissions to escalate their privileges to full Administrator status by assigning the Administrators group to any account without proper authorization checks.

Such unauthorized privilege escalation can lead to unauthorized access and modification of sensitive data, which may violate compliance requirements under common standards and regulations like GDPR and HIPAA that mandate strict access controls and protection of personal and sensitive information.

Therefore, exploitation of this vulnerability could result in non-compliance with these regulations due to potential unauthorized data access and administrative control.

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