CP Contact Form with PayPal <= 1.3.56 - Missing Authorization to Unauthenticated Arbitrary Payment Confirmation

The CP Contact Form with PayPal plugin for WordPress is vulnerable to Missing Authorization in all versions up to, and including, 1.3.56. This is due to the plugin exposing an unauthenticated IPN-like endpoint (via the 'cp_contactformpp_ipncheck' query parameter) that processes payment confirmations without any authentication, nonce verification, or PayPal IPN signature validation. This makes it possible for unauthenticated attackers to mark form submissions as paid without making actual payments by sending forged payment notification requests with arbitrary POST data (payment_status, txn_id, payer_email).