The Advanced Product Fields (Product Addons) for WooCommerce plugin for WordPress is vulnerable to Improper Input Validation in all versions up to, and including, 1.6.21. This is due to a logic flaw in the 'validate_cart_data' function. This makes it possible for unauthenticated attackers to bypass required paid addons and complete purchases at the base product price only, effectively stealing products by paying a fraction of the intended total. The vulnerability was partially patched in version 1.6.19.