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Verizon users might want to start looking for another provider. In an effort to better serve advertisers, Verizon Wireless has been silently modifying its users' web traffic on its network to inject a cookie-like tracker. This tracker, included in an HTTP header called X-UIDH, is sent to every unencrypted website a Verizon customer visits from a mobile device. It allows third-party advertisers and websites to assemble a deep, permanent profile of visitors' web browsing habits without their consent.Verizon apparently created this mechanism to expand their advertising programs, but it has privacy implications far beyond those programs. Indeed, while we're concerned about Verizon's own use of the header, we're even more worried about what it allows others to find out about Verizon users. The X-UIDH header effectively reinvents the cookie, but does so in a way that is shockingly insecure and dangerous to your privacy. Worse still, Verizon doesn't let users turn off this "feature." In fact, it functions even if you use a private browsing mode or clear your cookies. You can test whether the header is injected in your traffic by visiting lessonslearned.org/sniff or amibeingtracked.com over a cell data connection.How X-UIDH Works, and Why It's a Problem

To compound the problem, the header also affects more than just web browsers. Mobile apps that send HTTP requests will also have the header inserted. This means that users' behavior in apps can be correlated with their behavior on the web, which would be difficult or impossible without the header. Verizon describes this as a key benefit of using their system. But Verizon bypasses the 'Limit Ad Tracking' settings in iOS and Android that are specifically intended to limit abuse of unique identifiers by mobile apps.

Because the header is injected at the network level, Verizon can add it to anyone using their towers, even those who aren't Verizon customers.

The digital privacy world was rocked late Thursday evening when The New York Times reported on Securus, a prison telecom company that has a service enabling law enforcement officers to locate most American cell phones within seconds. The company does this via a basic Web interface leveraging a location API—creating a way to effectively access a massive real-time database of cell-site records. Securus’ location ability relies on other data brokers and location aggregators that obtain that information directly from mobile providers, usually for the purposes of providing some commercial service like an opt-in product discount triggered by being near a certain location. ("You’re near a Carl’s Jr.! Stop in now for a free order of fries with purchase!") The Texas-based Securus reportedly gets its data from 3CInteractive, which in turn buys data from LocationSmart. Ars reached 3CInteractive's general counsel, Scott Elk, who referred us to a spokesperson. The spokesperson did not immediately respond to our query. But currently, anyone can get a sense of the power of a location API by trying out a demo from LocationSmart itself. Currently, the Supreme Court is set to rule on the case of Carpenter v. United States, which asks whether police can obtain more than 120 days' worth of cell-site location information of a criminal suspect without a warrant. In that case, as is common in many investigations, law enforcement presented a cell provider with a court order to obtain such historical data. But the ability to obtain real-time location data that Securus reportedly offers skips that entire process, and it's potentially far more invasive. Securus’ location service as used by law enforcement is also currently being scrutinized. The service is at the heart of an ongoing federal prosecution of a former Missouri sheriff’s deputy who allegedly used it at least 11 times against a judge and other law enforcement officers. On Friday, Sen. Ron Wyden (D-Ore.) publicly released his formal letters to AT&T and also to the Federal Communications Commission demanding detailed answers regarding these Securus revelations.

Verizon says it will soon offer customers a way to opt out from having their smartphone and tablet browsing tracked via a hidden un-killable tracking identifier. The decision came after a ProPublica article revealed that an online advertiser, Turn, was exploiting the Verizon identifier to respawn tracking cookies that users had deleted. Two days after the article appeared, Turn said it would suspend the practice of creating so-called "zombie cookies" that couldn't be deleted. But Verizon couldn't assure users that other companies might not also exploit the number - which was transmitted automatically to any website or app a user visited from a Verizon-enabled device - to build dossiers about people's behavior on their mobile devices. Verizon subsequently updated its website to note Turn's decision and declared that it would "work with other partners to ensure that their use of [the undeletable tracking number] is consistent with the purposes we intended." Previously, its website had stated: "It is unlikely that sites and ad entities will attempt to build customer profiles.

However, policing the hundreds of companies in the online tracking business was likely to be a difficult task for Verizon. And so, on Monday, Verizon followed in the footsteps of AT&T, which had already declared in November that it would stop inserting the hidden undeletable number in its users' Web traffic. In a statement emailed to reporters on Friday, Verizon said, "We have begun working to expand the opt-out to include the identifier referred to as the UIDH, and expect that to be available soon." Previously, users who opted out from Verizon's program were told that information about their demographics and Web browsing behavior would no longer be shared with advertisers, but that the tracking number would still be attached to their traffic. For more coverage, read ProPublica's previous reporting on Verizon's indestructible tracking and how one company used the tool to create zombie cookies.