From Wikipedia, the free encyclopedia.
A web bug (also known as a tracking bug, pixel tag, Web beacon or clear gif
) is a technique for determining who viewed an HTML-based email message or a web page, when they did so, how many times, how long they kept the message open, etc.
Usually, a web bug is a transparent image or an image in the colour of the background of what you are viewing. It is typically 1*1 pixels in size. But other techniques can also be used to track usage, such as iframes.
In effect, most people won't notice that what they are viewing is bugged. Web bugs are a favorite tool that spammers use to verify working email addresses.
Web bugs usually have a unique name that somehow identifes what is being tracked. For example, let's name one http://nosey-site.example.com/mail_to_john_doe.gif.
When you read email sent by profit-motive.example.com, the HTML information in the message you are viewing causes your email software to automatically fetch the image, and thus give some information to nosey-site.example.com. Because they usually have a uniquely named bug in each message sent out (mail_to_john_doe in this case), they can see who read each message and when. This info can be used to measure the effect of advertisements, see when people look at pages, and enable them to look up the region the person is from via IP address information. Since many web bugs point to sites that aggregate information from hundreds or thousands of firms, and since HTTP cookies are often employed, they can be used to track people's browsing habits or even email usage across a wide swath of the Internet and can be used to accumulate personal information. Note that most aggregators deny that they use the information to invade the privacy of individuals.
Web bugs are also used on web pages when the author of a web page wants to allow another organization to also track people who use their page.
Some email software is designed to prohibit the display of remote graphics in HTML email by default, and thus prevent traditional web bugs from working. Examples include the Yahoo! and SpamCop/Horde webmail clients, and the Thunderbird, Opera, Mutt and Pine mail clients. But other HTML techniques like iframes can still be used to track email viewing, so some of these clients still aren't adequately protecting their users.
Plain-text email messages cannot contain web bugs. Avoiding the use of HTML when you forward mail will prevent others from being tracked by any web bugs that might be in the mail you are forwarding (e.g. you can convert them to plain text via copy-all and paste).
Disabling "third party" cookies will help prevent web bug users from linking together information on what email you read with the different web sites you visit, and thus from building a bigger personal information dossier...