The Sensei LMS WordPress plugin before 4.5.2 does not ensure that the sender of a private message is either the teacher or the original sender, allowing any authenticated user to send messages to arbitrary private conversation via a IDOR attack. Note: Attackers are not able to see responses/messages between the teacher and student
https://wpscan.com/vulnerability/5395d196-a39a-4a58-913e-5b5b9d6123a5