Java has further enhanced security to make the user system less vulnerable to external exploits. Starting with Java 7 Update 51, Java does not allow users to run applications that are not signed (unsigned), self-signed (not signed by trusted authority) or that are missing permission attributes. This means that you can get problems running an older applet in Java 1.7.51 and forward.
If you get “Missing required Permissions manifest attribute in main jar” when accessing certain homepages with an old applet – do the following:
1. Open up the Java Control Panel (Its an option in the standard windows control panel)
2. Click on the Security Tab in the Java Control Panel
3. Click on the Edit Site List button in the lower RH corner of the control panel
4. Add your URL to be allowed
5. Close out of the control panel.
6. You can now access your site through the browser window.
For more information: https://www.java.com/en/download/help/java_blocked.xml