WordPress GetPaid plugin: unauthenticated users can access and change other users' invoices

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.