Stream <= 4.2.0 - Missing Authorization to Authenticated (Subscriber+) Sensitive Information Disclosure via Heartbeat API

The Stream plugin for WordPress is vulnerable to authorization bypass in all versions up to, and including, 4.2.0. This is due to the plugin not properly verifying that a user is authorized to perform an action. This makes it possible for authenticated attackers, with subscriber-level access and above, to access all Stream activity records via the Heartbeat API.