On 13 December 2016, Apple released for OS X 10.10 Yosemite and Security Update 2016-003 for 10.11 El Capitan with patches for a number of vulnerabilities, including one that could allow an application to execute arbitrary code with kernel privileges and another that could enable a local user to cause a system denial of service.
On 17 January 2017, Apple pushed out a new version of to address a kernel issue that could cause a Mac running El Capitan to freeze and become unresponsive. If you applied Security Update 2016-003 before 17 January 2017,  will appear in Software Update. Be sure to make a backup before installing any of these updates in case of problems! (Free. For 10.10.5 Yosemite, 498.1 MB; for 10.11.6 El Capitan, 717 MB (623.9 MB for Supplemental); )