AT&T said Friday that it will phase out the use of so-called "super cookies" that track users mobile activity in a far deeper manner than they might have realized.
An AT&T spokeswoman told ProPublica that the super cookies have "been phased off our network."
AT&T did not immediately respond to PCMag's request for comment, but told ProPublica that the tracking was part of a test that is now done. The company might launch a similar program in the future, but if it does, customers will be able to opt out, the spokeswoman said.
Verizon is reportedly also experimenting with super cookies, but has no plans to stop, ProPublica said.
At issue are tracking cookies intended to serve up relevant ads. The practice is nothing new for Web users, but as the Electronic Frontier Foundation noted earlier this month, these "super cookies" focus on mobile surfing and users cannot easily opt out.
"It allows third-party advertisers and websites to assemble a deep, permanent profile of visitors' web browsing habits without their consent," the EFF said. "In fact, it functions even if you use a private browsing mode or clear your cookies."
The tracking tech is included in an HTTP header called X-UIDH, EFF said, but unlike traditional Web cookies, the X-UIDH "is tied to a data plan, so anyone who browses the Web through a hotspot, or shares a computer that uses cellular data, gets the same X-UIDH header as everyone else using that hotspot or computer."
"That means advertisers may build a profile that reveals private browsing activity to coworkers, friends, or family through targeted advertising," the organization said.
Forbes said last month that AT&T users could opt out of super-cookie tracking by going to a special URL on their mobile device (while connected to cellular, not Wi-Fi) and opt out (pictured).
According to EFF, Verizon's opt out option "does not actually disable the header. Instead, it merely tells Verizon not to share detailed demographic information with advertisers who present a UIDH value. Meaningful protection from tracking by third parties would require Verizon to omit the header entirely."
Verizon did not immediately respond to a request for comment.