Users of Microsoft's word processor have long been able to embed graphics in their documents, but versions of the program since Word 97 have allowed users to replace a bulky, memory-hogging graphic with a short URL pointing to the location of the graphic on a Web server. Each time the document is opened, the computer sends an HTTP request to the Web server asking for the graphic.
The Privacy Foundation reports that this can be used to track who is reading a document, without the reader's knowledge. Each time the graphic is requested, the Web server logs the IP (Internet Protocol) address and host name of the computer requesting it. By including a link to an almost invisibly small (1 pixel square) image, known as a Web bug, in their document, authors can collect this information on their Web server without their readers being aware that an image was requested.
"We are not aware of Web bugs being used in this way," said Richard M. Smith, chief technology officer of the Privacy Foundation. "But the possibility is troubling given the trend of using the Internet and monitoring software to track individual behavior."
Web bugs are so called because of their reported use by advertisers and others for discreetly tracking readers of Web pages.