Earlier today, Apple published a “critical” OS X security update that closes a potential vulnerability in the Network Time Protocol service.
As is typical of Apple and other software vendors, the specifics of the vulnerability are not mentioned in the App Store Updates tab. At the time we saw the update appear on our Macs, the Apple Security Updates web page had not yet been updated with the patch details.
The Network Time Protocol patch weighs in at 1.4MB and is available now via the OS X Yosemite App Store Updates tab.