When Apple beat Wall Street expectations convincingly on Monday after its best quarter ever, its share price fell. Financial analysts were worried about Apple's once stellar profit margins, the health of its irreplaceable chief executive, Steve Jobs, and fears that a slowing global economy will mean weaker sales of iPods, iPhones and MacBooks.

Amid this uncertainty, a different type of analyst told me of another troubling development for Apple, one that is probably not yet written into any financial models: Apple is beginning to attract the attention of hackers and malware writers.

A big factor in Apple's success in selling 2.5 million computers last quarter is growing user disaffection with Windows. Everything from recurring Vista headaches to security fears are driving Windows users into the Mac camp. Ironically, the resulting Mac sales are coinciding with – and causing – a new upsurge in malware written specifically for Apple users.

"It's still a drop in the ocean compared to Windows vulnerabilities, but [Mac vulnerabilities] have become more sophisticated and more criminally minded, rather than just proof of concept", Graham Cluley, senior technology consultant at the security company Sophos, says.

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  Bernhard Warner

The company reports today that two new Mac-ware Trojans that emerged in February and June ought to shake Mac users of their misconceptions that their computers (and, eventually, iPods and iPhones) are impenetrable. To put this in perspective, the first really pernicious piece of Mac malware emerged only in October, 2007, Mr Cluley adds, suggesting that a worrisome trend is about to get worse.

The piece of Mac malware identified in June, named "OSX/Hovdy-A Trojan", is the nastier of the two. It is capable of infiltrating a Mac, stealing passwords and opening the user's firewall to enable future exploits. If the modus operandi sounds familiar, that 's because a lot of the same virus gangs who perfected their exploits on Windows machines are now tweaking them for Macs, Mr Cluley adds.

Scarier still, the same tech-novice PC owners who failed to fortify their computers properly, allowing them to become spam relays and zombie DDOS attackers, are now making the switch to Macs. "I think the Mac user base will end up becoming polluted by some of the same people who have been infected time and time again in the Windows environment,” Mr Cluley says. “It's mainly the same people who buy a computer primarily to download porn and visit file-sharing sites."

For this reason, he says, "I think Apple will start to become a victim of its own success. I think hackers will see this community as a soft target."

The particularly craven hackers, phishing fraudsters and malware writers ply the trade for cash, not kicks. They follow the big fish, looking to infiltrate victims' machines to take advantage of their high-speed broadband connection and herd them into a botnet attack force. Or they aim to dupe them into turning over passwords and bank details. Until now, these miscreants have been content to target Windows users because there are more of them, making them more profitably prey. But as the Mac user base goes mainstream, cyber criminals will no doubt make the jump too.

It's a pattern that has security analysts uneasy. Apple will be hoping that it doesn't grow large enough to alarm the financial analysts.

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Bernhard Warner, a freelance journalist and media consultant, writes about technology, the internet and media industries. He can be reached at techscribe@gmail.com