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New research shows that ordinary Wi-Fi signals from smartphones can be used to “see” behind closed doors and track individuals in their own homes.

A group of scientists led by Yanzi Zhu at the University of California, Santa Barbara, have found a way to see through walls using ambient Wi-Fi signals and an ordinary smartphone.

They say the new technique allows an unprecedented invasion of privacy: “Bad actors using smartphones can localize and track individuals in their home or office from outside walls, by leveraging reflections of ambient Wi-Fi transmissions”, according to technologyreview.com.

This Wi-Fi based tracking technology looks for changes in an ordinary Wi-Fi signal that reveal the presence of humans. A Wi-Fi “vision” would make walls and doors almost transparent. Wi-Fi “light” is the signal coming out of transmitters, and people as well as other bodies can be tracked according to how they reflect this “light”.  

But this is not done by producing an image. The data that Zhu and colleagues use is just a measurement of the signal strength at a specific location. That doesn’t tell you anything about the location of the transmitter. And without knowing that, it’s impossible to say where any human that distorts the field would be.

So the first step in the researchers’ approach is to locate the Wi-Fi transmitter. They do this by measuring the change in the signal strength as they walk around outside the target building or room.

It is even possible to work out exactly where the transmitter sits inside a house, because floor plans of most homes and offices in the US are downloadable from places such as real estate websites. All this can be done via smartphone apps.

It’s not hard to imagine how a malicious actor might use this to work out if a building was occupied or empty.

The team say there are various defenses against this type of attack, such as geofencing Wi-Fi signals, but these are difficult to implement and have limited effectiveness. The most promising form of defense seems to be adding noise to the signals; the researchers are hoping to develop this in more detail in future.

In the meantime, this work suggests that the mere presence of Wi-Fi signals is a significant risk to privacy. “While greatly improving our everyday life, [wireless transmissions] also unknowingly reveal information about ourselves and our actions,” say Zhu and co. For the moment, this risk has been largely overlooked. That will need to change quickly.