Apple has released Mac OS X 10.7.2 and Security Update 2011-006, each of which fixes a large number of critical vulnerabilities in Mac OS X and Mac OS X Server, some of which may lead to unexpected application termination or arbitrary code execution. For more information, see http://support.apple.com/kb/HT5002
Mac OS X 10.7.2 update is available for Mac OS X 10.7.x and Mac OS X Server 10.7.x. Security Update 2011-006 is available for Mac OS X 10.6.8 and Mac OS X Server 10.6.8.
Software Update will install either Mac OS X 10.7.2 or Security Update 2011-006, whichever is appropriate for your system.