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.[ Internet Privacy: Cookies As a Weapon ]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 it's 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.But here's the irony: According to Mayer, while Turn recreates deleted tracking cookies for Verizon customers using the UIDHs, it doesn't do the same for opt-out cookies if those were set and later removed by those users."Clearing your cookies is not the way to opt-out of tailored advertising, and may in fact be counterproductive if you've cleared the cookie that indicates you have opted-out," Ochoa said. "That choice would then be erased and would need to be re-expressed."Verizon also allows subscribers to opt out of its Relevant Mobile Advertising (RMA) and Verizon Selects programs that use the UIDH. However, those settings just stop Verizon from sharing data associated with those user headers with advertising partners.The UIDHs are still injected into Web requests and can still be used for tracking or respawning cookies by companies like Turn or even those that are not members of the Network Advertising Initiative (NAI) and Digital Advertising Alliance (DAA)."So, what's a Verizon subscriber to do?," Mayer said. "Ad blocking would be effective, but it isn't supported by the stock Android or iOS browsers. Using a VPN or other secure proxy would work, though that's quite cumbersome. For an ordinary user, there simply is no defense.""It is clear that Verizon does not understand the privacy risks it is imposing on its customers," advocacy group the Electronic Frontier Foundation said in a blog post. "The UIDH program should be shut down today. Going forward, the company should undertake to obtain genuine prior, informed consent for any future tracking activities."