If you use a smartphone, there’s no telling what personal information it is leaking. What seems certain is that they are sharing personal data, without users’ knowledge. An investigation by The Wall Street Journal surveyed 101 popular smartphone apps for iPhone and Android models and found that 56 of them secretly transmitted the phone’s Unique Device Identifier (UDID) to other companies. Forty-seven also gave away the phone’s location. And five transmitted age, gender, and other personal details to outside tracking companies. One of the most popular offenders was the Pandora app, which sent age, gender, location, and phone identifiers to various online advertisers through iPhone and Android phones. "The great thing about mobile is you can't clear a UDID like you can a cookie," said one Internet ad network employee. "That's how we track everything."