Russian anti-virus vendor Doctor claims that the BackDoor.Flashback botnet has infected more than 550,000 computers running Mac OS X, most of which are located in the United States and Canada.

This once again refutes claims by some experts that there are no cyber-threats to Mac OS X.

"Over 550 000 infected machines running Mac OS X have been a part of the botnet on April 4, Doctor Web said. "These only comprise a segment of the botnet set up by means of the particular BackDoor.Flashback modification. Most infected computers reside in the United States (56.6%, or 303,449 infected hosts), Canada comes second (19.8%, or 106,379 infected computers), the third place is taken by the United Kingdom (12.8% or 68,577 cases of infection) and Australia with 6.1% (32,527 infected hosts) is the fourth. "

Systems get infected with BackDoor.Flashback.39 after a user is redirected to a bogus site from a compromised resource or via a traffic distribution system. JavaScript code is used to load a Java-applet containing an exploit, Doctor Web said.

According to some sources, links to more than four million compromised web-pages could be found on a Google SERP at the end of March. In addition, some posts on Apple user forums described cases of infection by BackDoor.Flashback.39 when visiting dlink.com.

The anti-virus vendor's data showed that attackers had begun to exploit vulnerabilities to spread malware in February 2012, and by March 16 they had switched to another exploit. The vulnerability has been closed by Apple on April 3 2012.

Of course, given that the company reporting these numbers is in the business of selling antivirus software, supporters of the 'virus-proof' OS X could claim that such a report was aimed at boosting Mac antivirus software sales.