MacDefender is getting worse day by day and is becoming a pain in the neck of Apple. MacDefender and its variants bypassed the latest security update (just after a few hours of its release) thrusted out by Apple on 31st may 2011. The Security Update 2011-003 was designed to detect and sweep MacDefender, the fake Mac antivirus program, and its variants viz. MacSecurity, MacGuard, and MacProtector.
The latest Mac malware variant comes hidden in mdinstall.pkg, an installer package that also contains data and time of Tuesday night at 9.24PM. See the image: 
This package installs MacGuard very quickly, the latest malware version, and infects the targeted machine without even requiring the admin password. The malware developers have been playing a goose-chase game with Apple, making Mac computers again test positive for the Mac malware.
Apple has reacted quickly by updating XProtect in order to scan and detect the behavior of the existing downloader. The Security Update 2011-003 contained modifications to the File Quarantine feature in the Mac OS X 10.6, aka Snow Leopard antivirus program. MacDefender was first noticed in early May by an antivirus company.
