Unseen visitors to your computer

Just because you're paranoid doesn't mean someone isn't following your every move online.

A startup called PubMatic is launching a new tool to help websites determine how many tracking files are being installed on users’ computers, The Wall Street Journal reports.

“The harm is that profiles [of users] are being built without the publisher knowing about it,” CEO Rajeev Goel says. The company using the hidden tracking files can then make money by selling the user’s profile to advertisers without the Web publisher’s knowledge, he said. And the files can slow the website down as well.

Mr. Goel said the number of tracking tools used varies widely depending on the website and the ad network involved. He said he sometimes sees as many as 10 to 12 invisible tools — often referred to as “pixels” or “beacons” — being placed via one ad.

A competitor, Rubicon Project, is testing a similar technology.