AT&T is phasing out its experimentation with supercookies, leaving Verizon as the only U.S. wireless carrier that's still tracking its customers on behalf of advertising agencies.

It's unclear when AT&T began implementing supercookies, but it was recently revealed that Verizon has been doing so for at least two years. AT&T said it was testing the tracking method on a limited scale, but now it has revealed that it is ditching supercookies.

Though Verizon's tracking initiative is still going strong, the consumer outcry that has prompted AT&T to at least reconsider its usage of supercookies shows that subscriber opinion can invoke meaningful change, according to Jacob Hoffman-Andrews, senior staff technologist at the Electronic Frontier Foundation.

"It is also a victory for carrier noninterference with customer data," states Hoffman-Andrews. "We call on Verizon to follow AT&T's lead and terminate their tracking header injection program or convert it to a true opt-in, immediately."

Hoffman-Andrews says there are reports indicating that wireless carriers outside the U.S. have been using supercookies to track their subscribers.

"We call on all network providers globally to respect their customers' data and not inject tracking headers," states Hoffman-Andrews.

Supercookies are a type of unique identifier header that are tagged to mobile devices at the network level, allowing smartphone and tablet users to leave digital fingerprints on each site they visit.

Traditional cookies are used to remind browsers of changes users made when visiting web pages, but they are only temporary and can be erased whenever an individual feels the need to do so. Because supercookies are placed at the network layer, it's impossible for consumers to erase them.

AT&T asserted that it reset its supercookies daily, which made it more difficult, but not impossible, for advertisers to compile browsing histories of individual mobile devices. Verizon says it also refreshes it supercookies, but it hasn't disclosed the frequency at which it does so.

While AT&T has been phasing out its use of supercookies, the company isn't scraping its system for the targeted delivery of ads. AT&T's Relevant Advertising program, as it calls it, is still active, but the company intends to give customers a true means for opting out of supercookies if it decides to bring the trackers back, according to AT&T spokeswoman Emily Edmonds.

"If and when we start a mobile Relevant Advertising program, customers will be able to opt out of receiving mobile Relevant Advertising, and also be able to choose not to have the associated numeric code inserted on their device," said Edmonds.

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