Sensei LMS 4.26.4 Broken Access Control via save_new_message

The Sensei LMS WordPress plugin before 4.26.4 contains a broken access control vulnerability in the legacy save_new_message handler hooked on the init action, which validates only a generic nonce without verifying course enrollment or restricting post_id to valid post types. Authenticated attackers, including unprivileged Subscribers, can supply an arbitrary post_id 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.