On Monday Apple issued a security update for Mac OS X Mountain Lion, Mavericks, and Yosemite that fixes a "critical security issue with the software that provides the Network Time Protocol service on OS X." Recommended for all users, Software Update suggests that you install the update as soon as possible. Details are currently unavailable, but Apple should issue them on the Apple Security Updates page shortly.
To access and install the update, choose Software Update from the Apple menu and click the Update button next to the latest Security Update entry. No restart is required.