Apple has released an update for its XProtect.plist malware definitions file which Apple calls the “safe downloads list.” (This is kind of a misnomer because it’s actually a list of unsafe downloads).
The latest update appears to block a new version of a Trojan horse that masquerades as a Flash Player update, which Apple labels OSX.FlashBack.B. Read more, including details on ways to get the definitions update, at http://security.thejoshmeister.com/2011/11/how-to-update-apples-safe-downloads.html .