Apple has released Mac OS X 10.6.6, which fixes a single critical vulnerability in Mac OS X 10.6.x and Mac OS X Server 10.6.x. This critical vulnerability could allow unexpected application termination or arbitrary code execution via a man-in-the-middle attack. For more information, see http://support.apple.com/kb/HT4498
The Mac OS X 10.6.6 update is available for Mac OS X 10.6.x and Mac OS X Server 10.6.x. Note that this issue does not affect systems prior to Mac OS X v10.6.