The Mac is no longer immune to Windows-like software exploits, and its users should prepare for a rougher year, a report from security vendor Sophos has said.

According to the Sophos Security Threat Report for 2008, the platform now has a critical mass that makes it worthwhile for malware writers to target it in parallel to Windows.

The company notes Mac-specific hacks such as the OSX-RSplug of November 2007, which redirected DNS queries to a compromised website as a typical example of the new species of Mac-hating malware. The novelty of the attack was that the site could detect whether the visitor was a Mac or PC, delivering a different attack to Apple users. Indeed the very popularity of browsers made it much easier for malware to be repurposed for different platforms.

“There are gangs out there who are now writing malware for Macs. That is a big shift,” said Graham Cluley of Sophos. “The conveyer belt has been started up for Apple. It is not yet an epidemic for Apple but it is a turning point,” he predicted.

“If enough Apple Mac users resist clicking on unsolicited weblinks or downloading unknown code from the web then there's a chance they could send a clear message to the hackers that it's not financially rewarding to target Macs,” he said.

He also suggested that the emerging class of Ultra-mobile PCs, such as the Asus EEE, which uses Linux, could in time fall prey to the same broadening of malware-writing horizons.

Sophos was now detecting 6,000 compromised websites a day – equivalent to one every 14 seconds – which made it almost impossible for users of any platform to protect themselves simply by not visiting certain types of websites. But only around 1 in 5 of these were hacker sites – the rest were legitimate sites that had been compromised without their owners’ knowledge, which rendered them especially dangerous.

The report mentions a long list of innocent-sounding examples, including art galleries, religious sites, and holiday property rental websites, as victims of malware compromise. Overall, most malware was still being written in China, the US, Russia, and a number of eastern European counties, with other European nations some way behind.

Worries over Apple security are nothing new, but have become more urgent in recent months, stating with Apple’s Leopard OS. Mac users have even had a taste of the same scareware – software that attempts to dupe users with bogus alerts - that rarely merits notice in the PC world.