According to a new study from Princeton University, televisions connected to the internet - which allow users to access streaming platforms like Netflix - use trackers to obtain data from customers.
In an email to The Verge, Arvind Narayanan - associate professor of computer science at Princeton - said: "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.
"There's very little oversight or awareness of their practices, including where that data is being sold."
It should be noted that the cheaper price of such TVs are partly subsidised by targeted advertising, which people technically agree to when they set up the device.
Researchers found trackers 69% of Roku channels and 89% on Amazn Fire, with data ranging from device type and city to Wi-Fi network and device serial number.
Narayanan added: "Some of these [trackers] are well known, such as Google, while many others are relatively obscure companies that most of us have never heard of."
In response, a Google spokesperson said: "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.
"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."