The Simple Newsletter Plugin WordPress plugin before 4.3.3 does not verify that the requester is the subscriber named in a public request before rendering that subscriber's stored details, allowing unauthenticated users to disclose a subscriber's personal data along with the key that authorises changes to their record.
https://wpscan.com/vulnerability/dd6e7f9c-b986-4bb7-afb4-530a8b149605/