The RegistrationMagic WordPress plugin before 6.0.9.9 does not validate a client-supplied quantity multiplier when calculating the total price of a paid registration, allowing unauthenticated users to register without paying and obtain an activated account holding the role the form grants.
https://wpscan.com/vulnerability/c8cdc20f-d6ca-4e02-b8ab-68cd9362952f/