After the virus has been activated by opening the game, the Cellery worm displays a message saying "Chancellery" and changes Windows settings allowing it to automatically run on the operating system.
While the game is running, the worm plays a MIDI music tune, but also searches for other networked computers to infect.
"This worm puts up the Tetris game as a smokescreen while it tries to hop from computer to computer across your network," said Graham Cluley, senior technology consultant for Sophos. "If your company has a culture of allowing games to be played in the office, your staff may believe this is simply a new game that has been installed -- rather than something that should cause concern."
Using games to fool people into activating viruses is an old trick, Cluley added. The Bibrog worm was disguised as a shooting game and the Coconut worm, written by Belgian virus writer Gigabyte, included a game where people could throw coconuts at photographs of security experts.
"Computer users can stay ahead of the game by ensuring they have kept their antivirus software updated," said Cluley. "With around a thousand new viruses being discovered every month it's essential that everyone has systems in place to defend their data and computer infrastructure."
By Wednesday lunchtime Sophos had received only a small number of reports about the Cellery worm.
ZDNet UK's Dan Ilett reported from London. For more coverage from ZDNet UK, click here.
