Other sites carrying on history sniffing include charter.net, espnf1.com,  feedjit.com and gamestorrents.com.

"We want to let the broad public know that history sniffing is possible, it actually happens out there, and that there are a lot of people vulnerable to this attack," says UC San Diego computer science professor Sorin Lerner in a written statement.

In addition to targeting potential phishing victims, history sniffing can be used by businesses to find out what competitors a site visitor has already checked out and by advertising firms trying to create user profiles so they can target them with ads of interest, the researchers say.

In some instances, the researchers found that Web site owners added the history sniffing code and in others the code was part of ads that were elements of the site pages. These external sources of sniffing code came from interclick.com, meaning tool.com and feedjit.com, all of which compile data on Web users' behavior for their customers.

Researchers also found that sniffing code on a Web page slows the load time for the page by 67%.