ETags are stored in a computer user’s browser cache so are unaffected when cookies are deleted. But the information these ETags contain allow for deleted cookies to be recreated so web tracking can continue.

After the researchers’ report was published on 29 July, Kissmetrics tweeted to say that people who wanted to opt-out of its tracking could do so using a free browser plugin called AdBlock Plus. But by the next day Kissmetrics had updated its privacy policy and posted an explanation of how its tracking works, saying that: “As of 30 July 2011, Kissmetrics uses standard first-party cookies.” Opt-out can now be achieved by setting an opt-out cookie.

One of the researchers behind the ETag report – Ashkan Soltani – also contributed to the 2009 report into the uses of Flash cookies to recreate deleted HTTP cookies, which led Quantcast to update its tracking methods.

Quantcast and a handful of other companies were sued in the wake of that report. Most of those complaints have since been settled or dismissed, but on Friday Kissmetrics was sued over its use of ETags.