Web 101: When Cookies Go Bad

We learned in the last post that cookies can make the web more useful and more fun. But cookies can have a darker side, too. Here we’ll learn about the seedier uses of cookies and what you can do about them.

If you already have a cookie from a website and you revisit that website, your browser only sends the information for that website’s cookie. So if you’re at Amazon, your browser only sends the Amazon cookie information and not the Ebay cookie information. There are a couple of exceptions to that rule, and this is where cookies cause problems for most people.

Lots of web sites contain advertisements. This isn’t normally a bad thing — the ads pay for the costs associated with the web site and make it possible for lots of free content to be out there for you. However, those ads are served up by a different domain than the site you’re on. If you’re visiting example.com, the ads might come from advexample.com and spamsalot.com. Both of those advertising domains can then put what are called “third-party cookies” into your browser. Later on, when you go to anotherexample.com, which also has an ad from advexample.com, that advertiser not only knows it’s you, but knows you recently visited example.com. These advertising cookies can track you across the web, and are the reason why after visiting a site selling shoes you suddenly start seeing lots of ads for shoes on other sites.

Some people really like having more targeted ads — if you have to see ads anyway, they may as well be for something you’re interested in. Other people find this tracking to be a bit creepy. If you’re in the latter group, there’s an easy fix. All the major browsers let you block any third-party cookies. (You can also use these same instructions to block all cookies.)

You probably already have a lot of tracking cookies stored in your browser. If you want to get rid of those, too, there are a couple of ways to do it. You can either go through your existing cookies one by one, or you can delete all your existing cookies. Deleting all your cookies is a lot faster than trying to figure out which ones you want and which ones you don’t, but it does mean you will need to sign in to any web sites again.

Deleting tracking cookies doesn’t stop all forms of tracking, but currently this is the most common form used. And now you have some control over the process.

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