An official source for security updates on Mac OS X. Users of Mac OS X can also get all their updates by selecting 'Software update...' from the Apple menu on the top left corner of the screen, or simply by waiting for the process to be performed automatically.
Quoted from the Apple site
This document outlines security updates for Apple products. For the protection of our customers, Apple does not disclose, discuss or confirm security issues until a full investigation has occurred and any necessary patches or releases are available. To learn more about Apple Product Security, see the Apple Product Security website.