A worm that targets critical infrastructure companies doesn't just steal data, it leaves a back door that could be used to remotely and secretly control plant operations, a Symantec researcher said on Thursday.

The Stuxnet worm infected industrial control system companies around the world, particularly in Iran and India but also companies in the U.S. energy industry, Liam O'Murchu, manager of operations for Symantec Security Response, told CNET. He declined to say how may companies may have be...