WC PayPay Gateway 0.5 - 0.9.3 - Unauthenticated Payment Bypass via Unverified Webhook

The Payment Gateway PayPay for WooCommerce WordPress plugin from 0.5 to 0.9.3 does not verify the authenticity of the payment notifications it receives before acting on them, allowing unauthenticated attackers who know the store's merchant identifier to mark arbitrary orders as paid, or to cancel or fail them.