A second Mac security threat has appeared.

Security experts at Sophos have issued protection against this second Mac OS X worm, which is called Inqtana-A.

This worm spreads among Macs using a Bluetooth vulnerability known as CAN-2005-1333. It appeared within days of the discovery of the Mac OS Leap-A Trojan last week.

Once again, the threat isn't critical. The Bluetooth flaw the worm exploits was addressed by Apple in mid-2005, when the company released a patch to protect against it.

This means the worm is unlikely to spread successfully.

Sophos warns that Mac users should keep their software up to date, applying Apple Security and OS (operating system) updates as they are released.

"It's disturbing to see a second worm for Mac OS X so soon after the first, but it should be remembered that this is only two compared with more than 100,000 viruses for Microsoft OSs," said Graham Cluley, senior technology consultant at Sophos.

"The good news is that Inqtana is not going to spread successfully in the wild, but this announcement will still be a shock to those in the Mac community who thought hackers were not interested in their OS."

Cluley stressed that Mac users cannot afford complacency. "They need to be just as careful about protecting their computers with antivirus software, firewalls and security patches as their friends and colleagues using Windows," he said.

This story first appeared on Macworld.co.uk.