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.