But Web bugs, named by a computer privacy specialist suspicious of them, can gather information without a sophisticated computer user's knowledge, since they set cookies and gather information even on pages displaying no ads. If there's an ad on the page, there's a suggestion this page has an affiliation with some other site. But the Web bug has no purpose but surveillance. Officials at the Federal Trade Commission, who said they learned details about Web bugs at a workshop this week about online profiling, will examine the bug's impact on consumer privacy, according to David Medine, the FTC's associate director for financial practices. That's because few sites disclose they are deploying a Web bug even if a user has set his Web browser to alert him if a site is trying to place a cookie.
Computer users can configure their browsers to block the setting of a cookie.
But if a user allows just one site in an advertising network to set a cookie, it enables all of the Web bugs in that network to perform. L.P. The code, on a page with stories about famous people who suffered from mental illness, sends information to an online advertising service called DoubleClick Inc., which gathers and analyzes information about computer users at some 1,400 Web sites. A spokesman for Janssen said its Web bug is used only to help the company identify the most popular material at the site. With the help of a cookie, the Web bug typically identifies a machine, the page it opened, the time it arrived and other details. That information, sent to a company that provides advertising services such as DoubleClick, can then be used to determine if someone subsequently visits another company page in the same ad network to buy something or read other material.