A Santa Claus worm is attempting to trick America Online, Microsoft MSN and Yahoo instant-messaging users into clicking on a file that delivers unwanted software to a victim's computer.

The IM.GiftCom.All worm attempts to dupe IM users into thinking an acquaintance has sent them a link to a harmless Santa Claus file, according to a security advisory issued Tuesday by IMlogic.

People who click on the file will see an image of Santa, but what they are less likely to notice is a so-called rootkit being installed onto their system. A rootkit is a tool designed to go undetected by the security software used to lock down control of a computer after an initial hack.

The malicious attacker can then distribute messages to the user's IM contacts, using a similar technique to lure the unsuspecting acquaintance to click on the link. The Santa worm is the latest tactic to be used on IM networks. Past tricks have included offers of movie clips to the latest release of "Star Wars" that instead led to an infected computer.