Visma Pay for WooCommerce < 1.2.2 - Unauthenticated Payment Bypass and Order Data Disclosure via Gateway Return Endpoint

The Visma Pay for WooCommerce WordPress plugin before 1.2.2 does not verify the authenticity of payment gateway return requests, relying on a merchant secret that is empty on a store which has not entered its credentials, allowing unauthenticated attackers to mark arbitrary orders as paid and to disclose the name, email address and postal address of any order's customer.