Munich, Aug 31 (DPA) It all used to be simpler for Mac users: viruses and other malicious software affected Windows computers, not them. That is slowly changing. 'The threats have intensified,' says Candid Wuest from anti-virus maker Symantec.

The biggest threat comes from hackers trying to sneak viruses onto computers. OSX.RSPlug.A is the name of one that preys on Mac users. It claims to be a video codec required to watch certain porno films, but in fact tries to redirect the surfer to a website forged to look like an eBay page. An attempt is then made to phish for the access data to the user's real eBay account.

'This trojan hasn't drawn much attention,' says Walter Mehl from Germany's Macwelt magazine. That may have to do with the fact that it is spread only through porn sites. It was also shoddily programmed.

Another trojan assumed control of Macs earlier this year, harnessing them into a 'Botnet', Wuest says. A botnet is a remote controlled network of computers employed by hackers to send out spam. Some 10,000 to 20,000 computers have been infected to date. That number, however, pales in comparison with the countless Windows PCs infected by the Conficker worm.

Windows is a more common target for attacks partially because it's user base is much larger. Virus writers know that with so many more computers running the software, even viruses that infect only a fraction of their targets can still rack up big numbers. The same is true for the search for vulnerabilities in applications - common programmes are a favourite target.

Malicious software used to be written only for specific operating systems.