Web pages use a publishing metaphor. They are pages, after all. We write, open, read and bookmark them. That's one reason why we assume that when a page downloads from a server, it's a one-way deal. The HTML describes the page, lays out the print, loads the graphics onto the page and into the cache.
There is at least the presumption of privacy. After all, reading is a personal (even an intimate) act. At times when interaction is required, such as when we fill out a form, there's a "submit" button that sends information back to the other end of the line. We're still in control.
And okay, we know about cookies and what they do. If we especially vigilant, we either refuse to accept them or go through the whole pile and weed out the suspicious ones.
But watch out. Big Brander is watching you. And not just with cookies.
It turns out that some companies are spying on you and your Web travels by using invisible 1x1 pixel transparent GIFs. These are in-line images downloaded from elsewhere, so the server-browser dialog can initiate covert reconnoissance on you and your subsequent surfage. You see no ad and suspect nothing. The cookie alert doesn't go off. But the bug -- as in bugging device -- has been planted.
Who's doing the planting? Usually a company that wants to learn something about you. Most of the time it's an advertising service that wants to "target" you with banners, email spam or whatever. But they can serve all kinds of purposes, known and unknown. Don Marti found one on a Fedex page. (Presumably they want to track customers the way customers want to track a package.) Richard Smith, the leading source of information on Web Bugs, found two on Quicken's home page, both to provide "hit" information to advertsing companies.
According to Smith's Web Bug FAQ <http://www.tiac.net/users/smiths/privacy/wbfaq.htm>, here is the information a Web Bug sends back to its server:
Of course any graphic can serve the same purpose. That's why the only way you can see a Web bug is to view a page's HTML source. But Smith also provides another way to at least discover what sites are playing slight-of-pixel games: The Web Bug Search Page <http://www.tiac.net/users/smiths/privacy/wbfind.htm>. It shows where each of ten bugging companies are sneaking in their little spies. Makes for interesting (and creepy) reading.
On the matter of security, Don Marti get's the last word: "When a site tries to violate users' common-sense expectation of privacy, it should be the system administrator's responsibility to protect the user unless the user requests otherwise. Web ad banners are a security hole."