A Web bug, also known as a Web beacon, is a file object that is placed on a Web page or in an e-mail message to monitor user behavior.
Unlike a cookie, which can be accepted or declined by a browser user, a Web bug arrives as just another GIF or other file object. It can usually only be detected if the user looks at the source version of the page to find a tag that loads from a different Web server than the rest of the page.
Although proponents of Internet privacy object to the use of Web bugs in general, most concede that Web bugs can be put to positive use, for example to track copyright violations on the Web.
According to Richard M. Smith, a Web bug can gather the following statistics:
- The IP address of the computer that fetched the Web bug.
- The URL of the page that the Web bug is located on.
- The URL of the Web bug.
- The time the Web bug was viewed.
- The type of browser that fetched the Web bug.
- A previously set cookie value.
Web bugs are often used by spammers to validate e-mail addresses. When a recipient opens an email message that includes a Web bug, information returned to the sender indicates that the message has been opened, which confirms that the email address is valid.