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Mobile devices could be compromising privacy by transmitting the device’s location history over the air, even when it is in sleep mode, new research found.

Of particular concern are newer Android devices, specifically those running Android 3.1 “Honeycomb” or later, said researchers at the Electronic Frontier Foundation (EFF). That version of the Google OS introduced a feature called Preferred Network Offload (PNO), which has a habit of broadcasting the names of the last 15 Wi-Fi networks a device joined, even when the screen is off.

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The idea is to conserve battery by allowing a phone to connect to known Wi-Fi networks even while in sleep mode, since Wi-Fi uses less power than the mobile data radio. The problem is your wireless network history can give a worryingly accurate and thorough picture of your movements, said EFF researchers.

“This data is arguably more dangerous than that leaked in previous location data scandals because it clearly denotes in human language places that you’ve spent enough time to use the Wi-Fi,” EFF’s Peter Eckersley and Jeremy Gillula wrote in a blog post. “Normally eavesdroppers would need to spend some effort extracting this sort of information from the latitude/longitude history typically discussed in location privacy analysis. But even when networks seem less identifiable, there are ways to look them up.”

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Networking history can also work as a general-purpose identifier. If a person is carrying a mobile device that just accessed the Wi-Fi networks at your home, your work, and your union hall, there’s a good chance that person is you. Even if you buy a new phone every week, as long as you keep connecting to Wi-Fi, snoops can spot you.

Not every Android device is susceptible. The EFF tested 28 handsets and found while several Google Nexus devices and several Motorola Droid models leaked Wi-Fi network data, other newer Android devices – including the Samsung Galaxy S3 and S4 – did not.

Neither did any of the iOS devices tested, including the iPhone 5s and 5c and going back to the iPhone 4 running iOS 6.

But the trouble is that without testing, there’s no way to know whether a particular device exhibits this behavior or not. For example, while the HTC One leaked Wi-Fi data, the HTC One Mini did not.

The EFF said it brought the issue to Google’s attention and a patch that fixes the issue has already at the Android Open Source Project. But it also points out it could be a long time before that fixed code makes it into a mainstream Android release.

And even though iOS looks to be in the clear – at least, as far as the recent versions that the EFF tested are concerned – the issue affects more platforms than just Android.

“Many laptops are affected, including all OS X laptops and many Windows 7 laptops,” the EFF said. “Desktop OSes will need to be fixed, but because our laptops are not usually awake and scanning for networks as we walk around, locational history extraction from them requires considerably more luck or targeting.”

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