Web bug

Overview

A web bug is any one of a number of techniques used to track who is reading a web page or e-mail, when, and from what computer. They can also be used to see if an e-mail was forwarded to someone else or if a web page was copied to another website. The first web bugs were small images.

Some e-mails and web pages are not wholly self-contained. They may refer to content on another server, rather than including the content directly. When an e-mail client or web browser prepares such an e-mail or web page for display, it ordinarily sends a request to the server to send the additional content.

These requests typically include the IP address of the requesting computer; the time the content was requested; the type of web browser that made the request; and the existence of cookies previously set by that server. The server can store all of this information, and associate it with a unique tracking token attached to the content request.

Web bugs are typically used by third parties to monitor the activity of customers at a site. Turning off the browser's cookies can prevent some web bugs from tracking a customer's specific activity. The web site logs will still record a page request from the customer's IP address, but unique information associated with a cookie cannot be recorded. However, web site server techniques that don't use cookies can be employed to help track a site's cookie-blocking users. For example, a web site can identify a request from a new visitor and send that visitor links that pass a unique ID as a GET parameter.

As an example of the way web bugs can make user logging easier, consider a company that owns a network of sites. This company may have a network that requires all images to be stored on one host computer while the pages themselves are stored elsewhere. They could use web bugs in order to count and recognize users traveling around the different servers on the network. Rather than gathering statistics and managing cookies on all their servers separately, they can use web bugs to keep them all together.

For e-mail, many web bugs be avoided by turning off HTML display and displaying only the text. Turning off the display of images while still using HTML is usually not enough since other techniques can still be used. One company advertises that they use over 20 different techniques to work around the security protections in most e-mail clients.

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