When Verizon bought AOL earlier this year, the two companies said they were teaming up to "pursue the joint vision of building the most significant media platform in the world." But they're also sharing something else — your personal data. Starting in November, Verizon will start sharing the information gathered by its controversial "supercookie" — an identifier inserted into mobile internet browsing activity as standard for customers of the network — with AOL's vast ad network. AOL's network, which is represented on 40 percent of websites, will be able to match internet users to their Verizon details, building profiles of their browsing habits and targeting them with specific ads based on their cellphone use.

The shift has worrying privacy implications because the tracking method, (often unblockable, undeleteable, and undetectable when inserted by carriers), can work like a beacon to let third parties follow you around the internet. Not only is the method invasive, but it's also unencrypted, meaning that outside sources could get their hands on it. In some cases, that could let the government spy on you — the NSA has historically used Google's "preferences" cookie to track web users, following them from 3G to Wi-Fi network based on the unique cookie ID on their phone. In others, it may allow companies to hit you with a bunch of ads tailored for your apparent tastes.

AT&T stopped using its own "supercookie" tracking code on its phones in November last year after a public outcry, but Verizon persisted, saying that it was "unlikely that sites and ad entities will attempt to build customer profiles." A few months later, it was revealed that ad company Turn was reviving tracking "supercookies" on Verizon customers' phones, even after they attempted to delete them. In response, Verizon said it would work with Turn to ensure that its use of the supercookie was "consistent with the purposes we intended."

Although the second-largest network in the US finally bowed to pressure in March and allowed customers to opt out of using the supercookie, it's still enabled as standard on Verizon phones, and a sizeable proportion of users are unlikely to know it exists. Karen Zacharia, chief privacy officer at Verizon, argued to Pro Publica that the sharing of your data was actually more privacy protective because "it's all within one company," but by combining Verizon's monitoring with AOL's almost incomprehensibly huge ad network, traffic tracking is getting more aggressive, less transparent and harder to escape.