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as explained here and lots of other cases , what is this tracking story ? browser vendors say we have same origin policy and for example google.com can only read cookies saved by google.com or read cookies that saved by google.com, if its true so how this tracking by google and advertising companies and other agencies works ?

Edit: please describe an example (java html etc) code to do a tracking

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3 Answers

If you want to see cookie tracking in action, try the Collusion plugin for Firefox. Here's a morning's browsing through sites like slashdot, a few security vendors and security stackexchange. See how they have indirect links.

Collusion

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Cookie tracking isn't really so much a "threat" as it is a privacy concern. Cookie tracking is generally done with full permission of the sites that you are being tracked across. The contents of a webpage can come from multiple websites as Tom indicated. In order to generate revenue or to gather analytic information about their visitors, the site you are visiting has agreed to put a link to a third party tracker's server. Your browser follows this link and supplies the tracking cookie to the company that does the analytics or advertising. Since the link is specific to the site you are coming from, the tracking service also knows where you were visiting.

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The contents of a Web page do not necessarily come from a single server. Web page from www.example.com contains a link <img src="http://ad.server.elsewhere.com/this-is-from-www-example-com"> which your browser will dutifully follow. That secondary server stores a cookie in your browser stating "I have been through www.example.com".

Later, you visit www.sample.org which contains a link <img src="http://ad.server.elsewhere.com/this-is-from-www-sample-org">. Your browser also follows that link, and since it is for the same server than previously (ad.server.elsewhere.com), the browser sends the previously stored cookie. Boom ! You've been tracked.

Happens every day. Happens right now. Happy Web surfing !

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in a practical example what is that ad.server.elsewhere.com which all sites have ? for example when i visit wikipedia or buy something from amazon or maybe any other thing online , how they track ? because there are creepy sites like techcrunch mashable etc which have lots of ad.server.elsewhere.com on them but most of good websites we visit don't have such things on their pages –  rathat Jan 29 '13 at 19:35
    
It is called "ads". Ads are pervasive, because, when you get down to it, that's what pays for the existence of the Web. Every site with ads is actually renting a bit of display space to a dedicated ad broker. –  Tom Leek Jan 29 '13 at 19:46
    
so if a site (for example stackexchange) don't place a universal ads like google adsense on its page we are safe right ? by the way what is that story in guardian about google and iphone ? if its part of web surfing so why they talk about iphone and safari so much loud ?! –  rathat Jan 29 '13 at 19:48
    
They talk about iPhone and Safari because it is Apple who makes the legal noise here. This is not a story about privacy -- by having an iPhone and surfing on the Web you pretty much surrender your privacy to both Apple and Google. This is a story about a specific tactical move in the long-standing war between Apple and Google. –  Tom Leek Jan 29 '13 at 19:53
    
somebody please negative vote to Tom's answer its not a good answer :) i have question why if XXX.com put <img src="YYY.com"> why browser must allow YYY.com collect information about XXX.com? if YYY.com need to save some cookie it must be saved by YYY.com and there is no need to give any information about XXX.com to YYY.com saved cookie because they are 2 different thing and there is no need share data with YYY.com , so please somebody negative vote to Tom and say correct answer :) –  rathat Jan 29 '13 at 20:19
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