Scientists discovered WiFi can spy on you—even when you’re not online

Here’s what you’ll learn when you read this story:

WiFi makes it possible to access the internet from anywhere, but now it can also be used for underhanded surveillance—and not in the ways you might think.

Sure, there are already countless bots that spy on your internet activity, no matter how you adjust your settings or what blockers you put up. But what’s especially unnerving about the new way WiFi is being used as spying tech is that it doesn’t rely on browsing history at all. It needs no specialized hardware. It doesn’t even need to reach your phone, tablet or any other gadget. You only need to walk past a location with an active WiFi network and radio waves WiFi sends out will bounce off you and create an image, opening up an opportunity to invade your privacy.

When WiFi signals propagate and interact with something or someone, the resulting patterns can be stitched together to create a sort of snapshot using radio signals instead of the light that a camera needs. Once captured and analyzed, these patterns create an image of you that can then be used to secretly follow you just about anywhere WiFi exists. Even if you don’t have your phone or laptop, WiFi surveillance can detect you based on solely on your radio-spectrum image. There are no special sensors involved. Tactics like this are problematic enough when individuals use them, so just imagine how they could be wielded by companies and governments that want to know your every move.

Beamforming feedback information (BFI) is what happens when devices linked to a WiFi network, such as smartphones and routers, send feedback signals within that network. Creepier still, this information is transmitted without encryption, meaning anyone in the range of that network can access it. Researchers Julian Todt, Felix Morsbach, and Thorsten Strufe of KASTEL Security Research Labs at the Karlsruhe Institute of Technology in Karlsruhe, Germany, are concerned. Earlier attempts at WiFi surveillance required LiDAR sensors or techniques that measured changes in read signals as they bounced off objects and people. BFI makes surveillance without consent much easier than other methods.

“With WiFi networks being ubiquitous in our everyday lives, the impact of unknown privacy threats is likely severe,” the researchers said in a study recently published in Proceedings of the 2025 ACM SIGSAC Conference on Computer and Communications Security. “[BFI] can infer the identity of individuals with very high accuracy, across different walking styles and perspectives, even with large sample sizes.”

As radio signals propagate, what’s received can be compared to what would be expected if the signals encountered no interference. That comparison offers an estimate of interference that can be helpful for correcting any errors in the data received. Naturally, these estimates carry information about the environment the signals were traveling through. Humans cause a significant amount of interference, and when that is analyzed, a human presence can be inferred. But the information that can be gleaned goes beyond just where you are. Interference analysis can find out sensitive information and also pick up on what you might be doing, allowing whoever is behind the surveillance to decide whether you seem suspicious, even if nothing remotely shady is going on.

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