Intego also recommends unticking your browser's option to open files after downloading. To avoid being stung by Mac Defender and the like, make sure you only download software from trusted sources, and never continue with installation if software starts trying to install itself. Those sites pop up warnings that a virus has been detected and automatically downloads the sketchy software. These scareware programs are spread through SEO (search engine optimisation) poisoning sites, which use SEO tricks to push websites to the top of search results. There are a number of other variations on the malware, including versions called Mac Security and Mac Protector. The cunning rascal! Basically it's like that bit in The Lion King 2 where Kovu's pride put Kiara in danger so Kovu can manufacture a brave rescue. If you enter your credit card details and pay for a subscription, the software stops with the porn, making you think it's defeated the non-existent virus. Once it's on your Mac it starts opening porn sites to make you think you have a virus, and informs you of nasty spyware where in fact there isn't any. Mac Defender looks like a well-designed, real piece of software, fooling users into installing it.

It's the first time these tactics have been employed to attack Macs, according to Intego, the Web security firm that identified the malware - attacks of this kind usually deliver dodgy Windows programs.

Mac Defender is fake antivirus software that dupes Mac owners into handing over their credit card details. Claiming to be antivirus software, Mac Defender and its variants pretend to clean up your computer - but all they're after is your cash. The old saw that Macs don't get viruses is under fire as a piece of malware called Mac Defender is rampaging across the Web.

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