Security experts are finding a growing number of viruses, worms, and Trojan horses that target cellular phones. Security researchers' attack simulations have shown that before long, hackers could infect mobile phones with malicious software that deletes personal data or runs up a victim's phone bill by making toll calls. The attacks could also degrade or overload mobile networks, eventually causing them to crash. And they could be even more insidious in the future by stealing financial data. Smart phones represent a particular risk. They offer Internet connectivity, function like minicomputers, and can download applications or files, some of which could carry malicious code. Mobile-device technology is still relatively new, and vendors have not developed mature security approaches. To counter the growing threat, antivirus companies have stepped up their research and development. In addition, vendors of phones and mobile operating systems are looking for ways to improve security.