Web Privacy Tips: Understanding "Cookies"

• What are Cookies?

  A "cookie" is a mechanism used by a web server to both store and retrieve information on a client machine. It is basically a block of data that is sent to your web browser (e.g., Netscape) from a web server. Your browser accepts this data and stores it on your hard disk. At some later time when you revisit a site, your web browser will give the information in the cookie back to the server.

  Cookies generally expire after a specified date. A cookie is never "executed" as code, so there's no danger of viruses infecting your system through the use of a cookie. But a cookie can contain whatever data the sender wants to store about you. It could contain your IP address or a unique ID generated by the server. If you provide an e-mail address or credit card number when you visit a site, this might be stored in the cookie.

• Uses for Cookies

  Cookies are typically used for things like tracking visits to web sites, site-personalization, on-line shopping carts, and web path mappings. Many commercial sites use cookies to collect information for marketing purposes or to track the number of unique visitors or to count repeat visits from an individual. You too can use cookies to collect information if you have a server that supports CGI scripts.

• Refusing Cookies

  If you are concerned about your privacy, you can prevent cookies from being placed on your hard drive. Most browsers, including Netscape Navigator and Microsoft Internet Explorer, contain an option you can set to warn you any time a server tries to send a cookie. You can then look over a cookie's contents before accepting or denying it. However, there are so many web servers now using cookies that it can become very annoying to accept or deny each one.
  
  Another approach is to force your browser to refuse all cookies by making the file where your browser stores its cookies nonreadable. Most systems have file-locking methods that you can use to do this (check the documentation that came with your system).
  
  Even if you refuse cookies, there are other ways a web server can track your visits. Most servers maintain logs that show your IP address and the URL of the previous site you visited as well as where you go next.

• More Information on Cookies

  To learn more about cookies check out these sites:
  

  • Andy's Netscape HTTP Cookie Notes
  • Persistent Client State HTTP Cookies

• Anonymous Surfing

  If you want to find out what a site knows about you when you visit, check out the CDT Privacy Demonstration at the Center for Democracy and Technology. This site contains good information for those who are concerned about privacy. If you want the opportunity to surf the web without revealing any personal information, check out The Anonymizer.

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Last updated: May 17, 2007