The Customer Engagement System WordPress plugin before 2.0.2 does not restrict access to its MailChimp interest-group lookup or validate the API key supplied to it, allowing unauthenticated users to make the server open connections to arbitrary internal hosts and ports and infer from response timing which ones are reachable.
https://wpscan.com/vulnerability/b91dc5fc-8b7b-4874-9e71-e3d07b41812a/