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"The Trojan is unusual in changing the computer's DNS server to new and malicious entry that actually intercepts and re-routes Web requests to malware-ridden and/or porn sites," said David Hobson, managing director for GSS.

"The problem for Mac OS X 10.4 users is that the user interface does not allow the owner to see that the DNS server has changed. Even under OS X 10.5, the change(s) are not obvious, as the added DNS entries are dimmed and cannot therefore be directly changed," he added.

According to Hobson, Apple Mac users should ensure they manually check and update their IT security software to the latest version from their IT security software vendor.

Hang on, that means that if your software is up-to-date you would still be immune. But isn’t that the same in the PC world? Aren't Macs immune even without third-party security software wrapped around them? Apparently not.

"They should also check with their software reseller that their products protect against this new and potentially serious malware development on the Apple Mac platform," said Hobson.

There’s more on the new Apple Mac Trojan here.