nexi Alpha Bank for WooCommerce <= 2.2.0 - Unauthenticated Payment Confirmation Forgery via Empty Shared Secret

The Payment Gateway WordPress plugin through 2.2.0 does not ensure a shared secret is configured before using it to authenticate the bank's payment-notification callback, allowing unauthenticated attackers to forge a valid notification and mark any WooCommerce order as paid on installs where the secret has not been set, and the same callback lets them move any pending order to a failed state whether or not the secret is configured.