The GetPaid plugin for WordPress before 2.8.63 contains an improper input validation...

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.