In a bid to beef up the security of Java applications, all versions of Java from v7 Update 51 will feature an enhanced security model making a user system less vulnerable to external exploits. This new security policy of Java will simply not allow users to run any java-application which is not signed (Unsigned), Self signed (not signed by trusted authority) or have missing permission attributes details in the manifest.
Since, this needs a fix on developers side users are simply blocked away from using Java applications which comes short in any of the above security considerations. I faced a similar issue where Dell's iDRAC 6 Virtual Console Client was denied execution by Java citing "Missing required Permissions manifest attribute in main jar" error. Since, there is no updated version of iDrac 6 Remote Console applet with fixed permissions we need to add an security exception for it as demonstrated below to let it run.