http://www.theregister.co.uk/2012/08/27/disable_java_to_block_exploit/tl;dr:
A browser based exploit for Java has been released on the web. It allows the distributor to use your browser's Java 1.7 runtime to download and execute anything it wants. For now, the only way to protect yourself is to disable the Java plugin within your browser. If you are using Java 1.6, you do not need to worry.
In Firefox: Press Firefox button -> Add-ons, go to Plugins and click the "Disable" button next to anything named "Java".
In Chrome: Type in: "chrome://plugins/" into the address bar (no speech marks). Scroll down to Java and click disable.
In Opera: Type in: "about:config" into the address bar. In the preferences editor, expand Java and uncheck "Enabled".