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 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 to well over 100,000 viruses for Microsoft operating systems," said Graham Cluley, senior technology consultant for 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 operating system."

Cluley stressed that Mac users cannot afford complacency: "Mac users need to be just as careful about protecting their computers with anti-virus software, firewalls and security patches as their friends and colleagues using Windows."