Verizon Wireless has been subtly altering the web traffic of its wireless customers for the past two years, inserting a string of about
50 letters, numbers, and characters into data flowing between these customers and the websites they visit.The company - one [of] the country's largest wireless carriers, providing cell phone service for about 123 million subscribers - calls this a Unique Identifier Header, or UIDH. It's a kind of short-term serial number that advertisers can use to identify you on the web, and it's the lynchpin of the company's internet advertising program. But critics say that it's also a reckless misuse of Verizon's power as an internet service provider - something that could be used as a trump card to obviate established privacy tools such as private browsing sessions or "do not track" features.