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POWhat identifying information can a website capture?
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copied!<p>If the owner of a web site wants to track who their users are as much as possible, what things can they capture (and how). You might want to know about this in order to capture information on a site you create or, as a user, to <em>prevent</em> a site from capturing data on you.</p> <p>Here is a starting list, but I'm sure I have missed some important ones:</p> <ol> <li>Referrer (what web page had the link you followed to get here). This is a HTTP header.</li> <li>IP Address of the machine you are browsing from. This is available with the HTTP headers.</li> <li>User Agent (what browser you are using). This is a HTTP header.</li> <li>Cookie placed on a previous visit. This is a header, available only if a cookie was placed earlier and was not deleted by the user.</li> <li>Flash Cookie placed on a previous visit. Some users turn off cookies, but <em>very</em> few know how to turn off Flash cookies. Works like a normal cookie although it depends on Flash.</li> <li>Web Bugs. Place something small (like a transparent single-pixel GIF) on the page that's served up from a 3rd party. Some third parties (such as DoubleClick) will have their own cookies and can correlate with other visits the user makes (for a fee!).</li> </ol> <p>Those are the common ones I think of, but there have to be LOTS of unusual ones. For instance, this:</p> <ol> <li>Time on the user's clock. <a href="https://stackoverflow.com/questions/13/determining-web-users-time-zone">Use JavaScript</a> to transmit it.</li> </ol> <p>... which I had never heard of before reading it here.</p> <hr> <p>ADDED LATER (after reading <a href="http://www.joelonsoftware.com/items/2008/09/15.html" rel="nofollow noreferrer">this</a>):</p> <p><strong>Please try to put just ONE item per answer, then we can use voting up to sort out the better/more-interesting ones.</strong> The list below is probably less effective.</p> <p>Ah well... NEXT time I ask a question like this I'll set it up better.</p> <hr> <p>And here are some of the best answers I got:</p> <ol> <li><a href="https://stackoverflow.com/questions/87365/what-identifying-information-can-a-website-capture#87413">James</a> points out that IE transmits the .NET framework version.</li> <li><a href="https://stackoverflow.com/questions/87365/what-identifying-information-can-a-website-capture#87454">AviewAnew</a> points out that one can <a href="http://ha.ckers.org/weird/CSS-history-hack.html" rel="nofollow noreferrer">find what sites you have visited</a>.</li> <li><a href="https://stackoverflow.com/questions/87365/what-identifying-information-can-a-website-capture#87529">Mecki</a> points out that Screen Resolution can be determined.</li> <li><a href="https://stackoverflow.com/questions/87365/what-identifying-information-can-a-website-capture#87529">Mecki</a> <em>also</em> points out that any auto-fill information your browser has cached can be determined, by creating a hidden field, then reading it with JavaScript.</li> <li><a href="https://stackoverflow.com/questions/87365/what-identifying-information-can-a-website-capture#87463">jjrv</a> points out that Flash can list the fonts on the user's machine.</li> <li><a href="https://stackoverflow.com/questions/87365/what-identifying-information-can-a-website-capture#87473">Kent</a> points out that you can find out <a href="http://weblogs.asp.net/jezell/archive/2006/08/25/Stealing-History.aspx" rel="nofollow noreferrer">what websites a person has visited</a>.</li> <li><a href="https://stackoverflow.com/questions/87365/what-identifying-information-can-a-website-capture#87493">Silver Dragon</a> points out you can determine the location of the mouse within the browsing window using Flash and AJAX.</li> <li><a href="https://stackoverflow.com/questions/87365/what-identifying-information-can-a-website-capture#87494">Jim</a> points out that you can tell what language the user has configured in their browser from a HTTP header.</li> <li><a href="https://stackoverflow.com/questions/87365/what-identifying-information-can-a-website-capture#87494">Jim</a> also mentions that you can detect whether people are using Greasemonkey or something similar to modify the page.</li> </ol>

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