Apple has released Mac OS X 10.10.3 and Security Update 2015-004. Each of these fixes a large number of critical vulnerabilities in Mac OS X and Mac OS X Server, the most serious of which may lead to unexpected application termination or arbitrary code execution. For more information, see http://support.apple.com/kb/HT204659.
Mac OS X 10.10.3 update is available for Mac OS X 10.10.x and Mac OS X Server 10.10.x.
Security Update 2015-004 is available for Mac OS X 10.9.5 and Mac OS X Server 10.9.5 (Mavericks), as well as for Mac OS X 10.8.5 and Mac OS X Server 10.8.5 (Mountain Lion).
Software Update will install either Mac OS X 10.10.3 (Yosemite) or Security Update 2015-004 (Mavericks and Mountain Lion), whichever is appropriate for your system.