The Simple Membership WordPress plugin before 4.7.7 does not verify that a PayPal payment notification was sent to the site's own configured merchant account before activating a membership, allowing unauthenticated users to activate or extend a membership using a payment made to an arbitrary PayPal account they control.
https://wpscan.com/vulnerability/82ac8cd9-db44-49c7-905c-16d261c5c516/