Coinbase Commerce Payment Gateway for WooCommerce <= 1.4.1 - Unauthenticated Webhook Signature Bypass via Empty-Default Webhook Shared Secret

The Coinbase Commerce Payment Gateway for WooCommerce WordPress plugin through 1.4.1 does not verify the authenticity of incoming payment webhook notifications, because the shared secret used to validate the request signature is empty by default and nothing rejects an unconfigured secret, allowing unauthenticated attackers to forge notifications that mark arbitrary orders as paid or cancel already-paid orders.