The Scheduler Widget plugin for WordPress is vulnerable to Insecure Direct Object Reference in all versions up to, and including, 0.1.6. This is due to the `scheduler_widget_ajax_save_event()` function lacking proper authorization checks and ownership verification when updating events. This makes it possible for authenticated attackers, with Subscriber-level access and above, to modify any event in the scheduler via the `id` parameter granted they have knowledge of the event ID.
https://plugins.trac.wordpress.org/browser/scheduler-widget/trunk/scheduler-widget.php#L158
https://plugins.trac.wordpress.org/browser/scheduler-widget/tags/0.1.6/scheduler-widget.php#L158