The Newsletters plugin for WordPress is vulnerable to privilege escalation in all versions up to, and including, 4.9.9.2. This is due to the plugin not restricting what user meta can be updated as screen options. This makes it possible for authenticated attackers, with subscriber-level access and above, to escalate their privileges to that of an administrator. Please note that this only affects users with access to edit/update screen options, which means an administrator would need to grant lower privilege users with access to the Sent & Draft Emails page of the plugin in order for this to be exploited.
The product does not properly assign, modify, track, or check privileges for an actor, creating an unintended sphere of control for that actor.