Just encountered this, too, when running an internal application after updating my JRE to 1.7u25. The warnings appear because of a new security feature introduced in 1.7u25 to prevent unauthorized code reuse. In my case, I was also presented with a dialog asking me to confirm that I wanted the application to have access to my PC.
If you have access to the jar file, add two attributes to it: Permissions and Codebase. You need to decide if the application requires access to everything on the PC, in which case you would use the value all-permissions for the Permissions attribute. Otherwise, use sandbox and the JRE will restrict the code's access level on the PC. Codebase probably needs to be the same as the codebase for the jnlp file, unless that jar file is being downloaded from a different URL, in which case it needs to be that URL.
Reference: Oracle's Java 7 documentation
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