The Japanized for WooCommerce plugin for WordPress is vulnerable to Improper Authentication in versions up to, and including, 2.8.4. This is due to a flawed permission check in the `paidy_webhook_permission_check` function that unconditionally returns `true` when the webhook signature header is omitted. This makes it possible for unauthenticated attackers to bypass payment verification and fraudulently mark orders as "Processing" or "Completed" without actual payment via a crafted POST request to the Paidy webhook endpoint.
