Just over a month after Comcast was caught inserting ads on websites its users visit while using the company's publicly accessible Wi-Fi hotspots, Wired is reporting that Verizon is also monitoring its users' mobile internet traffic.

As per the report, for past couple of years, the company has been inserting a string of about 50 letters, numbers, and characters called a Unique Identifier Header (UIDH) into its customers' web traffic, allowing it to build a detailed profile of users’ interests to facilitate customized advertising.

According to an AdAge report, Verizon calls the UIDH the PrecisionID. "It's a cookie alternative for a marketing space vexed by the absence of cookies", the report said, adding that corporate and government subscribers are excluded from the marketing solution.

Verizon reportedly changes the UIDH every week, and the information used by advertisers includes the subscribers’ postal address, device types, language preferences, gender, age, hobby, and personal interests.

“ISPs are trusted connectors of users and they shouldn’t be modifying our traffic on its way to the Internet”, said Jacob Hoffman-Andrews, senior staff technologist at the Electronic Frontier Foundation (EFF), who called the UIDH a “perma-cookie”, as it can be read by any web server Verizon customers visit.

As such, there is no way to disable the UIDH, Verizon says, adding that targeted ads would not be created if a user opts out of the company’s Relevant Mobile Advertising program. However, that doesn't necessarily guarantee privacy given that the unique identifier would still be broadcasted, and ad networks could start using it to create users' profiles even without their consent.

To check if your mobile device is broadcasting a UIDH, head over to this website. If there is no value in the Broadcast UID field at the top of the webpage then your carrier is not sending active tracking beacons to every web site you visit.

Verizon may not be the only company doing this. "Looks like AT&T has a similar header, and I've heard reports about Sprint", said Hoffman-Andrews.