The Electronic Frontier Foundation says Verizon's silent supercookies, which always follow subscribers around the internet, are being abused by creepy advertisers to push targeted ads.

The EFF says people should start using encrypted VPNs by default to claw back their privacy, because opting out of the system is not enough.

Two years ago Verizon started stamping a unique identifier token header (UIDH) on each website visit made by subscribers via its cellular data network. As the name suggests, the identifiers are unique to each person, allowing website owners to quietly build up profiles on people using these ID codes.

These records of online behavior are valuable to advertisers, as it allows them to get an idea of which adverts to display to each person: someone tracked across cycling websites will end up being shown ads for new bikes, for example.

Verizon allows people to "opt-out" of the system, meaning the telco won't allow advertisers to directly request and analyze your online wanderings, but the setting is mostly useless: every single HTTP request via its network is stamped with a UIDH regardless of the opt-out, and is thus visible to any web server one visits.

Now it appears that ad networks are using the UIDHs to monitor internet users without all that tiresome business of actually paying Verizon for the privilege, and since the system is baked in by the company there's very little people can do to stop them.

Code has already appeared on Github (since removed) that would allow anyone with the right setup to track Verizon's identifier, and reports are surfacing that Twitter has also managed to find out a way to follow the telco's clients online using the UIDH information.

The UIDH system is also pernicious in that it bypasses the anti-tracking measures in iOS and Android that are designed to protect mobile users' privacy: these measures tackle web cookies, rather than the specific UIDH HTTP header.

"It is possible to build an opt-out system that would stop this, but it would take a considerable amount of work and the current systems just can't do it," EFF staff technologist Jacob Hoffman-Andrews told The Register.

Stamping on a mandatory ID number on subscribers is such a nice idea that AT&T is also reportedly considering the same "feature" for its customers.

The only way to block the use of the UIDH system is to use a VPN and/or Tor for your online browsing. Tor is usually your go-to software for privacy but it can be difficult to set up on a mobile, but almost all smartphones have a VPN mode baked in and Hoffman-Andrews recommended users activate it to maintain online anonymity.

"The only way, in the short term, to stop this is if enough people complain about it," Hoffman-Andrews said. "Longer term, once we get encryption across the whole internet, this kind of thing will be less of an issue. But that's 10 or 20 years away at least." ®