Instead of blocking third-party cookies (which might be useful), why don't browsers allow them, but keep all cookies separate for each address-bar website?

For example, if a third-party website sets a cookie while the user is viewing a page on a certain website, then that cookie would only be visible to the third-party website when the user is visiting the same website.

This way, third-party cookies could still be set (as they may have legitimate uses), but tracking cookies would stop working.

  • Need any more info to help answer your question? Commented Aug 14, 2015 at 18:13

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Primarily because it would break things. For example, Facebook Like buttons would stop working because it wouldn't be able to check which Facebook account you are logged into. There are workaround where it shows a pop-up instead, but this is kind of ugly, and it also wouldn't be able to work as a passive page control (e.g. show number of number of likes and display comments).

Also, there is little advantage with additional complexity of maintaining a separate set of cookies depending on what the outer page origin is. I can't imagine a scenario where a 3rd party cookie would just need to keep track of the anonymous user when they are visiting a certain site. If this is required, then the outer site could simply pass a token to the 3rd party object.

If you want the additional privacy this would bring then you should simply disable third party cookies - this would effectively have the same result as you've suggested.

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