When trying to connect to an Avocent KVM switch you can run into all sorts of problems due to certificate issues and the amount of java privileges it demands. If you're using the IcedTea Java plugin that comes with Ubuntu you'll be able to log into the KVM interface, but it will not be possible to successfully run the jnlp file that loads the KVM viewer.

A fix is to grant the needed permissions via a java policy file in your home directory.

Note that you should really really remove the file after your KVM session since it removes most of the java security model.

?Create a file named .java.policy in your home directory with the following content:

grant {
   permission java.security.SecurityPermission
    "putProviderProperty.SunJCE";

   permission java.lang.RuntimePermission
        "accessClassInPackage.sun.security.internal.spec";

   permission java.lang.RuntimePermission
        "accessClassInPackage.sun.security.rsa";

   permission java.lang.RuntimePermission
        "accessClassInPackage.sun.security.internal.interfaces";
};

After this, restart Firefox and it should be able to load the KVM viewer.

In the RedHat BZ 471413 for this issue there is also a link to the java security settings:http://java.sun.com/developer/onlineTraining/Programming/JDCBook/appA.html

Maybe this could be used to widen permissions on a per-application level.
Short of that, every java app has ALL permissions as long as the little file is around. So remember to move it away if you don't work on the KVM switch.