Apple on Monday patched a security loophole on Mac OS with an automatic and seamless update. Apple spokesperson Bill Evans today told Reuters "The update is seamless" and "It doesn't even require a restart". The bug affected the network time protocol (NTP) which is a component used to synchronize clocks on all Macs OS X operating systems. Versions affected by the bug are:-

Mountain Lion v10.8.5

Mavericks v10.9.5 and 

Yosemite v10.10.1

To check if your Mac is already updated, type "what /usr/sbin/ntpd" in Terminal and you should get these NTP versions on it:-

Mountain Lion          ntp-77.1.1

Mavericks                   ntp-88.1.1

Yosemite                    ntp-92.5.1