According to researchers at University College San Diego, around 485 sites have been identified as using a bug in browsing software to track user habits.
The flaw allows a website to access all the websites previously visited and target ads. The bug also lets a website see if a user has visited rival websites.
The researchers said that browsers need better defences against websites finding out user information.
The survey scoured 50,000 of the most popular websites and unearthed 485 that carried a piece of code that checked a visitor’s browser history. 63 sites copied the data and 46 hijacked a user’s history.
Other websites tracked what users got up to on their own websites.
“Our study shows that popular Web 2.0 applications like mashups, aggregators, and sophisticated ad targeting are rife with different kinds of privacy-violating flows,” wrote the researchers.
The researchers have set up a website to let users check what information is leaking from there browsers.