Macs are still less likely than PCs to be exploited by malware, but Apple's rising popularity and Wednesday's discovery of a Mac-targeted Trojan could spell the beginning of the end for the Mac security haven.

Two years ago, Mark Borrie from the University of Otago in New Zealand, who manages more than 5,000 Macs, said Apple users were their own worst enemy when it came to security because they considered themselves immune from attacks.

His argument, like many security experts, was that the Mac OS faces fewer threats by virtue of its smaller footprint. At the same time he struggled to convince his staff that the lower threat did not equate to immunity.

Security experts have said that Mac-related malware would begin to emerge as OS X became more widespread but its global footprint has remained relatively small -- in 2006 it made up just 2.5 percent of worldwide PC sales, according to Gartner.

The story in the US is different though. Macs ranked third in Gartner's analysis of PC sales for Q3 2007 and were by far the fastest growing -- up 37 percent on last year's sales. With just eight percent market share, they were well behind leaders HP and Dell. Even so, Gartner predicts Macs will only make up 3 percent of all PC sales by 2011.

An even greater factor influencing the number of threats to the Mac OS, however is likely to come from Apple's portable devices rather than its laptops and desktops, say experts.

"Just looking at what's happening right now with the iPhone," F-Secure's security expert, Patrik Runald told ZDNet Australia, "it clearly shows there is interest in finding vulnerabilities in things running on full or stripped down OS X and as Apple's market share is growing that interest will only grow."

Even though the operating systems of Apple's devices are not the same, they are still based on BSD code -- a variant of Unix that OS X was originally based on, explained Simon Claussen, director of security vendor, PC Tools.

"Obviously there are more controls on [the iPod Touch and iPhone] than OS X for the computer, which means they're harder to exploit but not impossible to," he said.

The other appealing aspect of these devices is that they are generally always connected to a network and are typically owned by wealthy people, Claussen added.

Meanwhile, Symantec's Norton 360 spokesperson for Australia, David Hall, warned that the release of Leopard -- which has been shipped to two million users in just one week -- has brought a new challenge to Mac users: Bootcamp, the software that allows Windows to run on Mac hardware.

"Users need to understand that it's just running Windows on a piece of hardware, so they need to take the protection and install that on their machine," he said.