Apple has released Mac OS X 10.5.6, this update fixes a whopping 21 security bugs. You can download it over here, that page also contains more info about the fixed vulnerabilities.
This document describes the security content of Security Update 2008-008 / Mac OS X v10.5.6, which can be downloaded and installed via Software Update preferences, or from Apple Downloads.
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.