A new worm posing as a come-on to a Santa Claus site is traveling across all the major instant messaging networks, a security firm warned Tuesday, and when recipients visit the bogus site, they're infected with a file hidden from sight by a rootkit.

IMlogic said that the worm, dubbed "M.GiftCom.All," is circulating on the MSN, AOL, ICQ, and Yahoo instant messaging services, is a "Medium" threat, a relatively rare classification for the Waltham, Mass.-based company. Most IM worms and Trojans listed on its Threat Center receive only a "Low" classification.

Like virtually all IM worms, M.GiftCom.All includes a URL in messages it spams out to contacts hijacked from previously-infected PCs. When users naively visit that site -- which is billed as a harmless Santa site -- a file is automatically downloaded to their computers.

The file, usually named "gift.com," includes rootkit elements that cloaks it from security software. In addition, the downloaded executable tries to disable a number of anti-virus programs, adds a keylogger to the system to capture confidential information, and then spreads to others by snatching names from the user's IM client contact list.

IMlogic's alert can be read in full here.