Security Update 2015-007 & Others for OS X Mavericks & Yosemite Released
Apple has released Security Update 2015-007 for Macs running OS X Yosemite and OS X Mavericks, Safari 9.0.1 for OS X Mavericks and Yosemite, alongside a Mac EFI Security Update for some specific Macs. The security updates are recommended for Mac users to install, particularly if they’re not yet updating to OS X El Capitan.
OS X El Capitan users will find the OS X 10.11.1 update contains the same security fixes, and thus does not require a separate security update.
Mac users running OS X 10.9.5 or OS X 10.10.5 will find the updates available now in the Updates tab of the App Store, accessible from Apple menu > Software Update (or App Store). The downloads are reasonably small and require a reboot to complete. Nonetheless users should still start a backup of their Mac before proceeding with the updates.
The updates include many security related patches which you can read about here on Apple.com.
Some Macs also have a new EFI firmware security update available as well, labeled as Mac EFI Security Update 2015-002. Additional details about the EFI update can be found on the Apple knowledge base here.
Separately, Safari 9.0.1 includes minor fixes and security updates which can be read about here, and many users of OS X Mavericks and OS X Yosemite will find iTunes 12.3.1 available to download as well.