A researcher is advising drivers not to use a mobile app for General Motors Co's OnStar vehicle communications system, saying hackers can exploit a security flaw in the product to unlock cars and start engines remotely. "White-hat" hacker Samy Kamkar posted a video on Thursday saying he had figured out a way to "locate, unlock and remote-start" vehicles by intercepting communications between the OnStar RemoteLink mobile app and the OnStar service.