Apple has released Mac OS X 10.7.4 and Security Update 2012-002. Each of these 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/HT5281
Note that this update addresses the recent issue with legacy FileVault erroneously writing passwords in clear text to disk. See http://support.apple.com/kb/TS4272 for instructions on how to securely remove any of these remaining passwords form disk.
Mac OS X 10.7.4 update is available for Mac OS X 10.7.x and Mac OS X Server 10.7.x. Security Update 2012-0021 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 10.7.4 or Security Update 2012-002, whichever is appropriate for your system.