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Re: how does host B know that its visitor is the one that visited host A?

From: Linss, Peter <peter.linss@hp.com>
Date: Sun, 14 Aug 2011 06:47:22 +0100
To: Jonathan Rees <jar@creativecommons.org>
CC: "www-tag@w3.org" <www-tag@w3.org>
Message-ID: <0DE85D4B-1C69-4160-87AA-5EFEB10295D8@hp.com>
On Aug 12, 2011, at 8:11 AM, Jonathan Rees wrote:

> Probably everyone knows this but me...
> I shop at expedia.com (or somewhere) for a London hotel room. Later I
> visit guardian.co.uk and see an Expedia ad for London hotel rooms.
> I visit guardian.co.uk in a different browser (same computer & IP
> address but Safari instead of Chrome) and instead get an ad for
> magazine subscriptions. Apparently the Guardian can tell my two
> browsers apart somehow - it's using more than just my IP address to
> decide what ads to show me.
> How does this work? I.e. what are browser instances doing that leaks
> their identity to servers? Is it just a lucky guess based on
> User-agent or something?
> (a propos our privacy & tracking discussions)

For the cross-browser tracking they're likely using Flash Local Shared Objects. The fact that the info is shared among browsers is most likely a happy accident, the intent is more to have a "cookie" that doesn't get cleared when browser cookies are cleared and also often bypasses regular browser cookie controls and privacy modes.

Received on Sunday, 14 August 2011 05:49:13 UTC

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