The worm, Hagbard.A, tries to disguise itself on peer-to-peer networks as pirated downloads of the popular games titles "Grand Theft Auto: San Andreas," "Need for Speed Underground 2" and 400 other programs, Sophos said in an advisory released on Friday.
The downloaded program will copy itself to the file-sharing folder on the compromised PC and attempt to spread using the Windows Messenger. An instant message sent to others on the service contains a link to the worm and reads: "please download this...its only small brb."
"Because this worm can arrive in the form of an instant message, some users may be fooled into thinking it has come from a friend or colleague, rather than a virus on their PC," Graham Cluley, senior technology consultant for Sophos, said in a statement.
In addition, Hagbard puts a new twist on instant-messaging worms, security experts said, because it installs a Web server program on the infected computer. That could enable malicious attackers to gain remote access to data.
"This one has an interesting behavior," said Art Gilliland, director of product management at IMLogic, an instant-messaging security provider. "It downloads a Web server to give someone remote access, which is more malicious than the spyware or adware that gets downloaded onto systems from other IM worms."
More of these malicious IM worms may be in the works, as virus writers go from testing the technology to putting it into full-scale use, Gilliland added.
Although Hagbard carries a different bite than other worms, it has not spread as rapidly as other viruses, security experts said.
"We haven't seen a larger number of reports. But that may not be surprising, because most of our customers are enterprises," Cluley said, noting that Hagbard's prevalence is currently rated low. "If it does break out, it will likely be a problem with home users, because most companies prohibit file sharing."