You do really need a policy. Your employees expect IT to protect them, and your company's executives expect you to make sure that corporate data is protected from the things that employees do with their mobile devices. But your customers also want to know what you're doing with their data, and various contractors, distributors, suppliers and anyone else in your network need to know what they aren't allowed to do.
It's bad enough that a mobile device brings the same IT threats as any other network-connected device. It has full access to your LAN and can piggyback on whatever permissions you gave its owner. And of course, if it's being accessed by a naughty user, it can try to exceed that access.
But you really need a mobile-specific policy because mobile devices can be careless with all the data they store. They theoretically can track all movements. The microphone and camera can be activated remotely. Apps can access every phone call, email or text sent or received, as well as every site visited and every tweet tweeted. Some can even send messages under your name without your knowledge (No kidding. Even the Starbucks app has demanded the ability to tweet on customers' behalf). And some apps can identify every other app being used, along with a host of tech specs, like OS version, browser, serial number of phone, Wi-Fi particulars, carrier, etc.
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