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 orders customer.
The product does not sufficiently verify the origin or authenticity of data, in a way that causes it to accept invalid data.