Although folks are currently fixated on the Flashback botnet, Sophos draws attention to the fact that one in five Macs harbors some kind of Windows-oriented malware. The company gathered results over seven days from 100,000 Apple machines using its free antivirus program and 20% had one or more instances of Windows malware.

Sophos previously warned of this phenomenon. Last year, it tested 50 USB drives lost in public. Two thirds (33) were infected, including seven seemingly owned by Mac users. Naturally, Windows malware can't affect OS X, but Macs can infect PCs, so Windows users should still keep their guard up when accepting data from Mac users.

In the latest study, Sophos found that only 2.7% of the infected Macs contain malware actually harmful to them, with 75% of it being Flashback variants. Of the 20% harboring Windows malware, 12.2% carry Bredo, a three-year-old Trojan. Sophos notes that some machines contain malware samples dating back as far as 2007.

"Although most of the malware we're currently seeing on Macs is designed to infect Windows, you should still be a responsible member of society and ensure that you're keeping your Mac squeaky clean. Currently, Mac users are doing a pretty poor job on that front," the outfit said before offering a handful of common sense tips:

  • Run an anti-virus program, and make sure it's kept up-to-date.
  • Keep your security patches up-to-dated - not just for your operating system, but also for programs which you run on your Mac
  • Exercise caution about the programs you install, the links you click on and the attachments you open
  • Keep your wits about you and stay informed! If you keep clued-up about security threats you are less likely to be tricked by a cybercriminal into making a poor decision.