This article will provide information about Accessing web devices running
Unsigned or Self-signed Java applets
Starting with Java 7 Update 51, Oracle has enhanced their Java security model to make user systems less vulnerable to external exploits.
The new version of Java does not allow users to run applications that are not signed, Self signed (not signed by trusted authority) or applications that are missing permission attributes.
It is likely that older Java applets are not compliant with the security guide-lines implemented in Java 7 Update 51
This guide explains about the issue, and possible work-arounds.