A number of major security flaws in Java were revealed earlier this week, and Oracle has broken its normal slow update process for Java to fix them. Here's how to make your machine safe again.
As Oracle's blog post on the issue explains, you can avoid the problem by updating to the latest version of Java. Windows installations of Java include an automatic updater, so if you have that enabled you should be fine. If you want a speedy way to ensure you're protected, you can use Nineite's Java installer. On other platforms, you can download the updated version from Oracle.