WPFunnels <= 3.12.13 - Missing Authorization to Unauthenticated Arbitrary Product Price Manipulation via 'wpfnl_load_payment' AJAX Action

The WPFunnels plugin for WordPress is vulnerable to Missing Authorization in versions up to, and including, 3.12.13. This is due to the plugin registering the 'wpfnl_load_payment' AJAX action for both authenticated and unauthenticated (wp_ajax_nopriv_) users and the underlying add_offer_product_to_cart() function performing no nonce verification, no capability check, and no validation that the attacker-supplied product_id is the offer product actually configured on the attacker-supplied step_id. This makes it possible for unauthenticated attackers to add arbitrary WooCommerce products to a cart at any discounted price configured on any funnel step, enabling price manipulation and revenue loss at checkout.