Major websites have developed a new—and legal—technique to track users’ online activities, known as a “supercookie,” a powerful device that is impossible for computer users to detect, researchers at Stanford University and the University of California, Berkeley, said Wednesday. Supercookies are capable of re-creating user profiles even after people delete the regular cookies, researchers said. While many companies have been criticized for the selling of private data online, some sites that use supercookies said the supercookie tracking was inadvertent and they would cease using it. MSN and Hulu, who were both notified that they had been using supercookies, announced Wednesday they would immediately investigate the technique. Knowing Web-browsing history can provide advertisers with information about people’s interests, locations, and finances—and the federal government has debated ways to protect consumer privacy.