Web bug

A Web bug (also known as clear GIF or 1-by-1 GIF) is a tiny graphic image file embedded in a Web page, generally the same color as the background on which it is displayed, which makes it invisible to the naked eye. The Web bug sends back to its home server (which can belong to the host site, a network advertiser or some other third party): the IP address of the computer that downloaded the page on which the bug appears; the URL (Uniform Resource Locator) of the page on which the Web bug appears; the URL of the Web bug image; the time the page containing the Web bug was viewed; the type of browser that fetched the Web bug; and the identification number of any cookie on the consumer’s computer previously placed by that server.

Web bugs can be detected only by looking at the source code of a Web page and searching in the code for 1-by-1 IMG tags that load images from a server different than the rest of the Web page. In addition to disclosing who visits the particular Web page or reads the particular email in which the bug has been placed, in some circumstances Web bugs can also be used to place a cookie on a computer or to synchronize a particular email address with a cookie identification number, making an otherwise anonymous profile personally identifiable.

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