If you think back to late 2014 and early 2015, you might remember that AT&T and Verizon caught a lot of flak over something called a "supercookie." A supercookie is a tracking system that consumers can't disable.
The carriers used supercookies to track users' internet browsing over a cellular network so they could show targeted advertising and share customer information with third-party websites.
Obviously, consumers weren't happy about this. AT&T's first response was to charge customers a $30 fee to NOT track their data. Eventually, AT&T stopped using supercookies and claimed it had just been a "test" to see how well supercookies worked. Verizon, however, wasn't quite so consumer friendly.
Verizon's solution was to let its customers opt-out of the supercookie program. You just had to go into your Verizon account and disable "Relevant Mobile Advertising" and it would stop sharing your information with third parties. Unfortunately, there was, and still is, a big problem with the plan.
The way Verizon's supercookie works is to attach a unique identifying header, or UIDH, to your web traffic. Even if you opt out of the program and Verizon stops sharing your UIDH with third parties, the header is still there.
Any website or ad network you visit can record it and build up a profile of your web travels. Fortunately, that's finally changing.