The GetPaid plugin for WordPress before 2.8.63 contains an authorization bypass vulnerability that allows unauthenticated attackers to access and manipulate another user's invoice by supplying its ID through the maybe_use_invoice parameter in process_invoice(), bypassing the ownership and key checks performed by is_authorized_to_pay(). Attackers can submit a known pending invoice ID via maybe_use_invoice to read checkout pricing details and initiate a checkout workflow, resulting in an unauthorized state change of the invoice to on-hold.