Recently, it became known that OS Xis Screen Effects could be crashed with an extraordinarily long string of characters, giving full access to the desktop. A new Security Update from Apple aims to fix this vulnerability. From Apple:
Additional Info: Security Update 2003-07-14 addresses a potential vulnerability when a password is required upon waking from the Screen Effects feature, which could allow an unauthorized user access to the desktop of the logged in user.
The Security Update is available via the Software Update control panel, or from Appleis site. The update is a 1.7MB download, is good for both OS X 10.2.6 Server and Client, and requires a reboot once installed.