Microsoft has disabled an online tracking technology that a Stanford University researcher said allowed the company to track users on MSN.com, even after they deleted browser cookies and other identifiers.

Mike Hintze, Microsoft's associate general counsel, said the company took "immediate action" when it learned about the presence of so-called "supercookies" on its networks from Stanford University researcher Jonathan Mayer.

After Mayer identified Microsoft as one of several companies using supercookies for targeted advertising, the company investigated. "We determined that the cookie behavior he observed was occurring under certain circumstances as a result of older code that was used only on our own sites, and was already scheduled to be discontinued," Hintze said.

Mayer's research prompted Microsoft to move faster to disable the code, Hintze said. "At no time, did this functionality cause Microsoft cookie identifiers or data associated with those identifiers to be shared outside of Microsoft."

Super tracking

Supercookies are tracking mechanisms that do not rely on traditional browser cookies to store user browsing data. Examples of such cookies include Flash cookies in which user tracking data is stored in a little known Flash disk location, and cache cookies in which the data is stored in the entity tags (eTags) used by browsers as a bandwidth saving mechanism.

Such cookies are hard to get rid of, don't expire on their own and can store a lot of information, making them more appealing than traditional cookies to Internet marketers and web analytics firms. For instance, while an HTTP cookie stores just 4KB of data, Flash cookies can store up to 100KB.

One of the most controversial uses of such cookies has been to recreate or to "respawn' cookies that have been deleted by users.

Mayer said his research showed that Microsoft has code on its Live.com, MSN.com and its Atlas third party advertising networks that would have caused a user's cookie to be recreated even after it had been cleared by the user.

"It is difficult to estimate the number of users affected by Microsoft's respawning without knowing more about traffic to Microsoft's web properties and the conditions under which it would set [the identifier ID]," Mayer said in his blog. But the company had the ability to easily associate a user's interactions with msn.com, live.com and the Atlas network both before and after cookie clearing.

"One of the most prolific ad networks was using technologies that are widely frowned upon for circumventing user privacy choices," Mayer said. "At minimum this was a colossal privacy gaffe."

No protection

One problem with supercookies is that they are stored outside a browser, meaning they work outside browser privacy protections, said Ashkan Soltani, an independent security researcher and co-author of the UC Berkeley report. As a result, switching browsers to protect privacy doesn't help, Soltani said in a blog post.

"A Flash cookie acquired while using Firefox is also available to websites when using Internet Explorer," he said.

In many cases, such cookies are used without any user notice, opt-out or choice, Soltani said in an interview. Often, such cookies can be used by online tracking companies to peer into browsing habits across multiple sites to build a highly detailed profile about users, he said.