You've crushed your cookies. You've munged your e-mail. But still the spam streams into your in-box. Where's it coming from? One threat to privacy that you may not have considered is a little-known Web design trick called the Web bug.

Also known as clear GIFs, Web bugs are tiny, invisible graphic images that Internet marketers and advertisers implant on their Web pages to track which pages are being viewed and by whom.

Web bugs aren't always a threat to your personal privacy--many Web sites, including PCWorld.com, use them simply to monitor site traffic without identifying individual users or IP addresses.

When combined with cookies, customer databases, and other information-gathering methods, however, Web bugs can tell Web-site operators who you are, what sites you visit, and when you visit them. If that's information you'd like to keep to yourself, it's time to start hunting down and exterminating the bugs.

If you use ad- or cookie-blocking software, you may already be able to block Web bugs. Programs such as InterMute's AdSubtract (the SE version is free) and Guidescope's free Guidescope utility offer Web-bug-blocking features. And if your Web browser blocks third-party cookies or supports the P3P security standard, you may already be safe from Web bugs that track your personal browsing.

The Privacy Foundation's free Bugnosis utility flashes a visible or audible ("Uh-oh!") warning when it detects a Web bug in a page you're browsing (see Figure 1). The program is currently available only as an Internet Explorer 5. x add-on, but the forthcoming Outlook and Outlook Express versions of Bugnosis will be able to detect Web bugs in HTML-format e-mail messages, which are the same as Web pages.

You can install Bugnosis in a few minutes, even over the slowest of connections. The Privacy Foundation's FAQ at the Bugnosis page explains more about how Web bugs work, and why you should care.

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