Big security issue on both PC and MAC using Java. You should disable Java when you browser internet and always keep Java updated. If you do not know what Java is just disable it until a website ask for it.

If you need Java for a Web site, use two-browser. If you normally browse the Web with Firefox, you can consider disabling the Java plugin in Firefox, and then using an another browser (Chrome, IE9, Safari, etc.) with Java enabled to browse only the site(s) that require(s) it.

Here is how you disable on Firefox and Internet Explorer in pictures and a link

Here is the last link for updates