Apple has issued patches for a very serious security hole that affects OSX 10.9 (Mavericks) and iOS devices (iPhones, iPods, iPads, Apple TV). You can test if you’re affected by browsing to this test site.
The patch for Mac OSX 10.9 users is to upgrade to 10.9.2. You can do that through Apple Menu/Software Update or downloading and installing the 10.9.2 Combo Updater. Note that either way you’ll have to restart your machine.
The patch for iOS devices can be found by upgrading to iOS 7.0.6 or 6.1.6 in the System Settings/General/Updates settings. Note that devices that can run iOS 7 but are still on iOS 6 may have to go to iOS7 to get the fix. Yes, that is annoying, as some of us have avoided it on our older iPhones etc since it can make the devices run slower.
I recommend doing this soon as you can.