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If I disable third-party cookies in my web browser is this likely to cause me many issues when browsing the web? In particular, do they have common uses other than advertising?

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It's possible that you might have issues if you disable third party cookies.

See below:

When viewing a Web page, images or other objects contained within this page may reside on servers besides just the URL shown in your browser.

While rendering the page, the browser downloads all these objects. Most modern websites that you view contain information from lots of different sources. For example, if you type www.domain.com into your browser, widgets and advertisements within this page are often served from a different domain source. While this information is being retrieved, some of these sources may set cookies in your browser.

First-party cookies are cookies that are set by the same domain that is in your browser's address bar. Third-party cookies are cookies being set by one of these widgets or other inserts coming from a different domain.

Source: http://en.wikipedia.org/wiki/HTTP_cookie#Third-party_cookies

In other words, disabling third party cookies might render some web page elements useless, if they are loaded from a "third party", that is, from another site than the page a user is visiting.

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I know third party cookies are used by third party elements, the question is how many of them need cookies? Will it break almost all, many, a few? –  Casebash Jun 5 '10 at 22:33
    
Surely not all needs cookies. But anything that has some kind of functionality (e.g. polls, login providers like OpenID, Google, etc, etc) will break. A suggestion: Disable the third party cookies in your browser, and visit your favorite sites for an hour or two. That will give you a hint. –  TFM Jun 6 '10 at 8:12
    
aren't 3rd-party cookies disabled by default on Safari? I never heard that all/most sites that have "some kind of functionality" are broken for Safari. –  Kip Mar 20 '12 at 2:54
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There's some interesting discussion of this on the Mozilla bug-tracking software, in this request to disable 3rd-party cookies by default in future versions of Firefox. One quote from a Firefox dev:

... in practice [disabling third-party cookies] doesn't help all that much. Advertisers have long gotten around this through redirects and hosting ads in frames. You can easily test this by setting the "no 3rd party" pref and the "ask me about cookies" pref and see all the requests you get for advertizer's cookies anyway. Or just check your cookies and see all the advertisers in there that you have never directly visited.

Withholding cookies during redirects or from frames breaks lots of legit sites, so the "no 3rd party cookies" option is mostly a feel-good measure but doesn't accomplish much.

However, Safari (at least on mobile) disables third-party cookies by default, and the internet doesn't seem to be broken from my iPhone. (Although there are ways for advertisers to get around the restriction, and even Google has been caught doing so.)

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