Updates to Apple’s Mac operating system has always required user intervention to install the update through the Mac App Store or intentionally allow updates to occur automatically (Apple Menu –> System Preferences –> App Store).
Apple has had the ability to perform automatic OS updates for over two years but has always chosen the ‘user intervention’ route. Until now. Macs running OS X Mountain Lion v10.8.5, OS X Mavericks v10.9.5 or OS X Yosemite v10.10.1 should have been updated to patch a critical flaw in the Network Time Protocol (NTP). This is used to keep your system clock in sync. A flaw allows a remote user to execute arbitrary (and nefarious) code that could cause buffer overflows.
If you are running Mountain Lion, Mavericks or Yosemite, you may want to verify that the update has occurred. If it hasn’t, you should install the update (Apple Menu –> App Store –> Updates) right away.