Robokassa payment for WooCommerce < 1.9.0 - Unauthenticated Arbitrary Order Status Change to Failed

The Robokassa payment gateway for Woocommerce WordPress plugin before 1.9.0 does not verify the authenticity of an incoming payment-notification request before acting on the order named in it, allowing unauthenticated attackers to force any WooCommerce order into a failed state. Order identifiers are sequential, so every order in the shop can be taken out of the fulfilment queue one request at a time.