Add smart TVs to the growing list of home appliances guilty of surveilling people’s movements. A new study from Princeton University shows internet-connected TVs, which allow people to stream Netflix and Hulu, are loaded with data-hungry trackers.

“If you use a device such as Roku and Amazon Fire TV, there are numerous companies that can build up a fairly comprehensive picture of what you’re watching,” Arvind Narayanan, associate professor of computer science at Princeton, wrote in an email to The Verge. “There’s very little oversight or awareness of their practices, including where that data is being sold.”

Of course, data is part of the reason TVs have gotten so cheap. Today, Roku’s sell for less than $200, subsidized in part by targeted advertising. Technically, people agree to have their data sold when they set up their devices. But many aren’t aware it’s even happening.

Technically, people agree to have their data sold when they set up their devices

That’s true for other smart home technology, too. In a different study, researchers at Northeastern University looked at 81 smart home devices and found that some, including Amazon’s Ring doorbell and Alexa, and the Zmodo doorbell, monitor when a user talks or moves, even when they’re not using the device. “The app used to set up the [Ring] device does not warn the user that the doorbell performs such recording in real time, the doorbell offers no indication that recording is occurring, and the only disclosure is in fine print as part of the privacy policy,” the paper says.

To understand how much surveillance is taking place on smart TVs, Narayanan and his co-author Hooman Mohajeri Moghaddam built a bot that automatically installed thousands of channels on their Roku and Amazon Fire TVs. It then mimicked human behavior by browsing and watching videos. As soon as it ran into an ad, it would track what data was being collected behind the scenes.

Some of the information, like device type, city, and state, is hardly unique to one user. But other data, like the device serial number, Wi-Fi network, and advertising ID, could be used to pinpoint an individual. “This gives them a more complete picture of who you are,” said Moghaddam. He noted that some channels even sent unencrypted email addresses and video titles to the trackers.

The study found trackers on 69 percent of Roku channels and 89 percent of Amazon Fire channels

In total, the study found trackers on 69 percent of Roku channels and 89 percent of Amazon Fire channels. “Some of these are well known, such as Google, while many others are relatively obscure companies that most of us have never heard of,” Narayanan said. Google’s ad service DoubleClick was found on 97 percent of Roku channels.

“Like other publishers, smart TV app developers can use Google’s ad services to show ads against their content, and we’ve helped design industry guidelines for this that enable a privacy-safe experience for users,” a Google spokesperson said in a statement emailed to The Verge. “Depending on the user’s preferences, the developer may share data with Google that’s similar to data used for ads in mobile apps or on the web.”

Both Roku and Amazon Fire allow users to turn off targeted advertising. But doing so only stops a user’s advertising ID from being tracked — not the other uniquely identifiable information.

“Better privacy controls would certainly help, but they are ultimately band-aids,” Narayanan said. “The business model of targeted advertising on TVs is incompatible with privacy, and we need to confront that reality. To maximize revenue, platforms based on ad targeting will likely turn to data mining and algorithmic personalization/persuasion to keep people glued to the screen as long as possible.”