It follows the Flashback trojan attack which security experts believe infected up to half a million Macs.

BBC: Apple releases Flashback patch for older OS

The Flashback Trojan that infected 700, 000 Macs at its peak earlier this month represents a rude awakening for Apple users who long believed their computers to be immune from the kind of malicious software that infects PCs.

FORBES: Connect

Last week, a Russian security company reported that as many as 600, 000 Macs, most of them in the United States and Canada, had been infected by Flashback, a Trojan horse virus that infiltrates computers, secretly giving someone else remote access to them.

CNN: Apple: Update will fix Mac 'Flashback' virus