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8.2 Some Problems with Cookies

Providing convenience to the user and added value to the site owner is the purpose behind cookies. And despite much misinformation, cookies are not a serious security threat. Cookies are never interpreted or executed in any way and thus cannot be used to insert viruses or attack your system. Furthermore, since browsers generally only accept 20 cookies per site and 300 cookies total and since each cookie can be limited to 4 kilobytes, cookies cannot be used to fill up someone's disk or launch other denial of service attacks.

However, even though cookies don't present a serious security threat, they can present a significant threat to privacy. First, some people don't like the fact that search engines can remember that they're the user who usually does searches on certain topics. For example, they might search for job openings or sensitive health data and don't want some banner ad tipping off their coworkers next time they do a search. Even worse, two sites can share data on a user by each loading small images off the same third-party site, where that third party uses cookies and shares the data with both original sites. (Netscape, however, provides a nice feature that lets you refuse cookies from sites other than that to which you connected, but without disabling cookies altogether.) This trick of associating cookies with images can even be exploited via e-mail if you use an HTML-enabled e-mail reader that "supports" cookies and is associated with a browser. Thus, people could send you e-mail that loads images, attach cookies to those images, then identify you (e-mail address and all) if you subsequently visit their Web site. Boo.

A second privacy problem occurs when sites rely on cookies for overly sensitive data. For example, some of the big on-line bookstores use cookies to remember users and let you order without reentering much of your personal information. This is not a particular problem since they don't actually display the full credit card number and only let you send books to an address that was specified when you did enter the credit card in full or use the username and password. As a result, someone using your computer (or stealing your cookie file) could do no more harm than sending a big book order to your address, where the order could be refused. However, other companies might not be so careful, and an attacker who got access to someone's computer or cookie file could get on-line access to valuable personal information. Even worse, incompetent sites might embed credit card or other sensitive information directly in the cookies themselves, rather than using innocuous identifiers that are only linked to real users on the server. This is dangerous, since most users don't view leaving their computer unattended in their office as being tantamount to leaving their credit card sitting on their desk.


FOXTROT © 1998 Bill Amend. Reprinted with permission of UNIVERSAL PRESS SYNDICATE. All rights reserved

The point of all this is twofold. First, due to real and perceived privacy problems, some users turn off cookies. So, even when you use cookies to give added value to a site, your site shouldn't depend on them. Second, as the author of servlets that use cookies, you should be careful not to use cookies for particularly sensitive information, since this would open users up to risks if somebody accessed their computer or cookie files.

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