The Sensei LMS WordPress plugin before 4.26.4 contains a broken access control vulnerability in the legacy savenewmessage handler hooked on the init action, which validates only a generic nonce without verifying course enrollment or restricting postid to valid post types. Authenticated attackers, including unprivileged Subscribers, can supply an arbitrary postid for any course, lesson, or quiz to create a private message addressed to that post's author, bypassing the enrollment checks enforced by the newer REST send-message controller.