Evil malware writers have tweaked the MacDefender Trojan to no longer need a password for installation if a Mac user is running the default administrator account, finally popping the bubble of invulnerability Mac users have enjoyed and MacDailyNews savaged me over a few years ago.
Similar to the RogueAV malware on PCs that falsely informs end-users that their computers are infected with all manner of nasties, MacDefender claims to be security software that is instead attempting to extort a payment from you to clear the fake infections and to stop a slew of filthy porn pop-ups polluting the screens of 'infected Mac users.