Apple's Xprotect for OS X was introduced as part of OS X Snow Leopard in 2009 as a rudimentary anti-malware system that allowed Apple to block installation and running of the relatively rare cases of known malware targeting Mac systems. By 2011, Apple had upgraded the system to allow users' Macs to automatically check for updated definitions every 24 hours, ensuring up-to-date protection.
While primarily targeted specifically at malware protection, the Xprotect system also allows Apple to enforce minimum versions of plug-ins such as Flash and Java, and the company has on occasion blocked older versions of both of these plug-ins to ensure that users upgrade their systems.
Apple's use of the Xprotect system has, however, declined in recent months, perhaps signaling a slowdown in malware and other security issues affecting Mac systems. The most recent significant update to Xprotect definitions prior to yesterday came in mid-September to address the OSX/Leverage.A malware.