Run unsigned Java applets

Reference How do I control when an untrusted applet or application runs in my web browser? :

Starting with Java 8 Update 20, the Medium security level has been removed from the Java Control Panel. Only High and Very High levels are available.

The exception site list provides users with the option of allowing the same applets that would have been allowed by selecting the Medium option but on a site-by-site basis therefore minimizing the risk of using more permissive settings.

You may have to restart your browser for the settings to take effect.