The Pie Register – User Registration Forms. Invitation based registrations, Custom Login, Payments WordPress plugin before 3.7.0.1 does not sanitise the invitaion_code GET parameter when outputting it in the Activation Code page, leading to a reflected Cross-Site Scripting issue.
https://wpscan.com/vulnerability/f1b67f40-642f-451e-a67a-b7487918ee34