Known as session-replay scripts, these were found on sites like Microsoft, IBM, The London Telegraph, and numerous others.
The scripts record keystrokes, mouse movements, and scrolling behaviour and send the full content of pages viewed to third parties.
"Unlike typical analytics services that provide aggregate statistics, these scripts are intended for the recording and playback of individual browsing sessions, as if someone is looking over your shoulder," researchers Steven Englehardt, Gunes Acar, and Arvind Narayanan wrote.
Collection of page content could lead to the leaking of information about medical conditions, credit card details, and other personal information, the researchers pointed out.
Passwords were often included in session recordings and sensitive user input was redacted in an imperfect way, they said.
Additionally, the researchers found that commonly used ad-blocking lists like EasyList and EasyPrivacy did not block many of these scripts.
A full list of the 482 sites that were found to be using these scripts is here.