Just because your Web browser is set to block third-party tracking cookies that doesn't mean all of them are being blocked.

A growing number of Web sites are quietly resorting to the use of "first-party," subdomain cookies to skirt antispyware tools and cookie blockers and allow third-party information gathering and ad serving, according to some privacy advocates and industry analysts.

Though the cookies are not fundamentally different from other third-party cookies, they are very hard to detect and block, said Stefan Berteau, research engineer with CA's antispyware research team. The result: companies could theoretically use the cookies to quietly gather and share consumer information with little risk of detection, he said.

So far, the use of first-party, subdomain cookies appears to be less prevalent than standard third-party cookies, Berteau said. "But it's the kind of thing that might catch on quickly."

The growing, but largely hidden, issue of online consumer-tracking and information-sharing burst into the open in recent days because of the controversy generated by Facebook's Beacon ad-serving technology. In that case, the use of tracking technology was acknowledged by the company, though it has been blasted for not allowing users to easily opt out and for failing to disclose how extensively it was being used.

First-party, sub-domain cookies are those that appear to be served up by the primary Web site a user is visiting; in reality, they are being issued by an external third party. For example, a company whose primary domain name is xyz.com could create a sub-domain called trackerxyz that falls within the xyz.com domain so it would look like this: www.trackerxyz.xyz.com

This subdomain actually points to a third party's server. But because the parent domain names are the same, the user's browser sees that server as belonging to the parent -- and treats cookies from both equally.

Web sites that allow such cookies are taking advantage of the fact that the standards used to categorize cookies rely on domain names, not IP addresses, Berteau said. In other words, whether a cookie is seen as a first-party cookie or a third-party cookie depends on the domain from which the cookie was served up, not on the IP address of the server itself. "Basically a sub-domain can be pointed to any IP address" while still having its cookies treated as first-party cookies, he said.

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