Browser fingerprinting is one possible way.<p><a href="https:&#x2F;&#x2F;panopticlick.eff.org&#x2F;" rel="nofollow">https:&#x2F;&#x2F;panopticlick.eff.org&#x2F;</a><p>[edit: clarified that this is a solution, I have no knowledge as to whether that is what is done.]