OpenMage magento-lts before 20.18.2 Payment Bypass via Free Payment Method

OpenMage magento-lts before 20.18.2 and 21.0.0-beta1 before 21.0.0-beta3 accepts the free (No Payment Information Required) payment method on orders with a non-zero total because the zero-total check runs only when payment data is imported. An API user with cart permissions can select free via cart_payment.method while the total is zero, raise it with cart_coupon.remove, and submit cart.order to place an unpaid full-price order.