Apple today released a security update for OS X El Capitan and Yosemite users. Security Update 2016-001 and 2016-005 comes in at around 400MB and does require a restart. For security reason, Apple cannot disclose exactly what has been fixed in this update. However, you can view some of the fixes here.
To get the update, users can go to the Updates page in the Mac App Store. You can also manually download it from Apple’s download page.