Visa Acceptance Solutions <= 2.1.0 - Unauthenticated Authentication Bypass via Billing Email

The Visa Acceptance Solutions plugin for WordPress is vulnerable to Authentication Bypass in all versions up to, and including, 2.1.0. This is due to the `express_pay_product_page_pay_for_order()` function logging users in based solely on a user-supplied billing email address during guest checkout for subscription products, without verifying email ownership, requiring a password, or validating a one-time token. This makes it possible for unauthenticated attackers to log in as any existing user, including administrators, by providing the target user's email address in the billing_details parameter, resulting in complete account takeover and site compromise.