Dishonest antivirus software which tricks users into buying or installing it, usually by infecting a user's computer, or by pretending the computer is infected.
MacDefender may also be called MacSecurity, MacProtector, MacGuard or MacShield.
This rogue is being distributed via SEO-poisoned websites.
The user is then asked to download the installation package for a program (i.e., the rogue program) that will 'fix' the detected infection.
The rogue is installed in the Applications directory, as per normal Mac applications. It also adds its own menulet to the desktop menu bar.
Image 1: application icon used by the Mac Defender rogue
The rogue is also added to the Login Items for the user, so that it will automatically launch every time the user logs in.
Image 2: MacDefender added to Login Items during instsallation
The rogueware poses as a legitimate virus scanner, reporting bogus infections and occassionally opening pornographic websites to trick users into believing an infection is present on the computer.
Image 3: Dialog displayed when MacDefender is run on a (clean) test machine
Image 4: Notifications displayed during a 'scan'. Note the menulet in the menu bar
This maybe be a further tactic to push the user into paying money for the bogus application, or to steal the user's credit card information.
For more information about MacDefender, see the related Labs Weblog post: