If you're using OS X & have Quicktime installed (a likely scenario), you'll want to get this update. The file weighs in at just under 26MB, & requires that you're running 10.3.9 or later. Grab the download here, or read about the patch here.

"A QuickTime movie containing a maliciously crafted Quartz Composer object could give an attacker access to local data and send it to an arbitrary Web location, the Apple posting said."