A group of computer security experts say they figured out how to hack the keyless entry systems used on millions of cars, meaning that thieves could in theory break and steal items without leaving a broken window. Remote entry systems on millions of cars made by Volkswagen since 1995 can be cloned to permit unauthorized access to the car's interior. Another system used by other brands including Ford, General Motor's Opel and Chevrolet and Renault can also be defeated. In a paper to be delivered Friday at the Usenix security conference in Austin, Texas, the authors say a thief could use commonly available equipment to intercept entry codes as they are transmitted by radio frequency, and then use that information to clone another remote so the car could be opened. The paper leaves out key details on how to perform the hack but says the codes can be intercepted with commercially available equipment. "It is unclear whether such attacks ... are currently carried out in the wild by criminals," the report says. "However, there have been various media reports about unexplained theft from locked vehicles in the last years." The report did not establish the exact number of cars that use the vulnerable systems. The report authors said that insurance companies might have to accept that car theft scenarios that would otherwise be considered insurance fraud have a higher probability of being genuine. The only surefire countermeasure, they said, would be to stop using the remote and fall back on the mechanical lock using the conventional metal key.