Internet security specilaist Sophos claims to have discovered the first ever virus for the Apple Mac OS X platform. The virus, named OSX/Leap-A (also known as OSX/Oompa-A) spreads via instant messaging systems.

The OSX/Leap-A worm spreads via the iChat instant messaging system, forwarding itself as a file called latestpics.tgz to contacts on the infected users' buddy list. When the latestpics.tgz archive file is opened on a computer it disguises its contents with a JPEG graphic icon in an attempt to fool people into thinking it is harmless.

The worm uses the text "oompa" as an infection marker in the resource forks of infected programs to prevent it from reinfecting the same files.

The news has sent ripples of disquiet through the Apple user community that has long basked in the idyllic belief that the Mac OS X environment was invulnerable to malware.

"After the possibility was raised with Renepo-A in 2004, some Mac owners have refused to accept that OS/X is capable of harbouring viruses and malware, so Leap-A may come as an extremely nasty shock," said  Sean Richmond, senior security consultant for Sophos Australia and New Zealand. "The fact that OSX/Leap-A is spreading to unprotected Mac OS X systems is proof that there is a real malware threat for Macs - users can no longer think 'It's not going to happen to me!'"

Experts at Sophos are continuing to examine OSX/Leap-A and will issue further information shortly. Sophos says its customers have been automatically protected against the worm since 23:25 EST, 16 February 2006.

"This is the first real virus for the Mac OS X platform," continued Richmond. "Apple Mac users now need to take the same security precautions as Windows users."

Some members of the Apple Macintosh community have claimed that OSX/Leap-A is a Trojan horse and not a virus or worm, because it requires user interaction (the user has to receive a file via iChat and manually choose to open and run the file contained inside).

However, this is not the definition of a Trojan horse, according to Sophos.

A Trojan horse is a seemingly legitimate computer program that has been intentionally designed to disrupt and damage computer activity.

Trojan horses do not replicate or have any mechanism of spreading themselves. They have to be deliberately planted on a website, or accidentally shared with another user, or spammed out to email addresses. There is nothing inside a Trojan's code to distribute themselves further to other victims.

Trojan horses do not contain any code to distribute or spread themselves, viruses and worms do.

OSX/Leap-A is programmed to use the iChat instant messaging system to spread itself to other users. As such, it is comparable to an email or instant messaging worm on the Windows platform. Worms are a sub category of the group of malware known as viruses.

Therefore, it is correct to call OSX/Leap-A a virus or a worm. It is not correct to call OSX/Leap-A a Trojan horse.