A company that correlates data about users across different websites to share with marketers is using unique IDs inserted by Verizon into mobile Web traffic to recreate tracking cookies that have been deleted by users.
The story began a few months ago when it was reported that both Verizon and AT&T were injecting unique identifiers in the Web requests of their mobile customers. Privacy activists criticized the practice because it creates so-called perma-cookies (permanent cookies) that cannot be deleted by users as they're added en-route, at the carrier level.
AT&T later said that it was only testing the system and has since stopped, but Verizon continues to add UIDHs (Unique Identifier Headers) to Web traffic as part of two programs that allows advertisers to identify users based on demographics, interests, location and other criteria.
"When the ad partners see the identifier, they can determine that the device is part of a group an advertiser is trying to reach and then serve the right advertisement," the company explains on its website.
However, the company believes its unlikely that advertisers will use its UIDHs to build individual customer profiles because the UIDH changes frequently and "other permanent and longer-term identifiers are already widely available in the wireless area and could be used to build customer profiles."
However, Jonathan Mayer, a computer scientist at Stanford University, discovered that one advertising company called Turn, which tracks users across the Web when they visit major sites including Facebook, Twitter, Yahoo, BlueKai, AppNexus, Walmart and WebMD, uses the Verizon UIDH to respawn its own tracking cookies.
This means that if a Verizon mobile subscriber deletes the cookies stored in his browser, including the one created by Turn to track his activity across different websites, the company will recreate the same cookie and resume the tracking when it later sees his Verizon UIDH. This means that the user won't get a clean slate and his tracking profile will increase over time.
"The privacy impact also goes beyond individual mobile browsers," Mayer explained in a blog post Wednesday. "If a Verizon customer tethered with their phone, their notebook could get stuck with the zombie value. (The ultimate in cross-device advertising!) And the zombie value could spread between cookie stores on a device, including between the web browser and individual apps. (The ultimate in inter-app advertising!)"
Turn admits that it is using Verizon's UIDH to recreate deleted cookies, but the company's general counsel and chief privacy officer, Max Ochoa, argued in a blog post that "clearing a cookie cache is not a widely recognized method of reliably expressing an opt-out preference."
According to Ochoa, users who want to opt out from behavioral advertising should instead use the consumer opt-out tool from the websites of the Network Advertising Initiative (NAI) or the Digital Advertising Alliance (DAA). This online tool can be used to create opt-out cookies for each of the advertising companies participating in those self-regulatory programs, including Turn.
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