GetPaid WordPress Plugin < 2.8.63 Negative Quantity Payment Bypass

The GetPaid plugin for WordPress before 2.8.63 contains an improper input validation vulnerability that allows unauthenticated attackers to bypass payment processing by submitting a negative item quantity on a public payment form. Attackers can exploit the unchecked quantity value in process_item() to produce a negative line subtotal that reduces the invoice total to zero, causing process_free_payment() to mark the invoice as paid without any actual gateway charge.