Zac Hall, 9to5Mac:
Apple today released an OS X NTP Security Update for Mac users running 10.10 Yosemite, 10.9 Mavericks, and 10.8 Mountain Lion. The update is recommended for all users and fixes a “critical security issue with the software that provides the Network Time Protocol service for OS X,” according to Apple. Check the Updates tab of the Mac App Store to grab the security update.
No restart required. Here's Apple's support document: About OS X NTP Security Update