Payment Button for PayPal <= 1.2.3.41 - Missing Authorization to Unauthenticated Arbitrary Order Creation

The Payment Button for PayPal plugin for WordPress is vulnerable to unauthorized order creation in all versions up to, and including, 1.2.3.41. This is due to the plugin exposing a public AJAX endpoint (`wppaypalcheckout_ajax_process_order`) that processes checkout results without any authentication or server-side verification of the PayPal transaction. This makes it possible for unauthenticated attackers to create arbitrary orders on the site with any chosen transaction ID, payment status, product name, amount, or customer information via direct POST requests to the AJAX endpoint, granted they can bypass basic parameter validation. If email sending is enabled, the plugin will also trigger purchase receipt emails to any email address supplied in the request, leading to order database corruption and unauthorized outgoing emails without any real PayPal transaction taking place.