Verizon has been using a kind of tracking cookie on its network that can’t be deleted. The company has insisted that the special cookie is only there for its own use and said that “it is unlikely that sites and ad entities will attempt to build customer profiles” using the cookies. However, a new report from Pro Publica indicates that Verizon’s promise has already come undone.

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Apparently, an online ad clearinghouse called Turn — which is used by Google, Yahoo, Facebook and others — has figured out how to piggyback on Verizon’s invincible cookies to track users.

“Turn is taking advantage of a hidden undeletable number that Verizon uses to monitor customers’ habits on their smartphones and tablets,” Pro Publica explains. “Turn uses the Verizon number to respawn tracking cookies that users have deleted… Some users try to block such tracking by turning off or deleting cookies. But Turn says that when users clear their cookies, it does not consider that a signal that users want to opt out from being tracked.”

Verizon, for its part, tells the publication that it’s looking into Turn’s use of its special cookies and isn’t ready to make judgments about the firm’s activities yet.

For a more detailed description of how this whole process works, check out Pro Publica’s full report by clicking the source link below.