Freemius is like a marketplace for WordPress Developers, where they can sell premium or free versions of their plugin. Freemius then takes care of the payment processing and licensing of the plugins, as required.
The Freemius Framework is what the plugins use to communicate to the Freemius platform to ensure anyone using the plugin, has the right authorisation to do so.
The problem is with plugins that haven't update to the latest version of the framework, leaving themselves open to what's known as a CSRF attack, via AJAX calls. In this case essentially an attacker could gain access to log files or initiate debug mode.