In the Ninja Forms Contact Form WordPress plugin before 3.4.34.1, low-level users, such as subscribers, were able to trigger the action, wp_ajax_nf_oauth, and retrieve the connection url needed to establish a connection. They could also retrieve the client_id for an already established OAuth connection.
https://wpscan.com/vulnerability/dfa32afa-c6de-4237-a9f2-709843dcda89