Custom User Registration Fields for WooCommerce <= 2.2.3 - Unauthenticated Privilege Escalation via 'afreg_select_user_role' Parameter in Store API Checkout

The Custom User Registration Fields for WooCommerce plugin for WordPress is vulnerable to Privilege Escalation in versions up to, and including, 2.2.3. This is due to the plugin accepting an attacker-controlled afreg_select_user_role value from the unauthenticated WooCommerce Store API /wc/store/v1/checkout request in the af_reg_checkout_data_to_order_meta_data_block() function, persisting it in order meta, and then passing it directly to WP_User::add_role() in the af_reg_custom_order_processing_function() function (hooked to woocommerce_thankyou) without validating against the plugin's admin-configured allowed role list. This makes it possible for unauthenticated attackers to elevate their privileges to Administrator by creating an account during checkout with a modified JSON body specifying administrator (or any other role slug) as the desired role. Note: The exploit requires the "User Role Selection" setting to be enabled.