A computer scientist studying at Stanford exposed the practice on Tuesday, writing in a blog post that while at the Dulles Airport he noticed AT&T placing popup and banner ads on websites, including those from educational organizations and the government. This obviously makes for a bad web browsing experience, but the bigger problem is that it introduces security issues, since websites aren't expecting to be tampered with like this.

The practice of serving extra ads to people using an open Wi-Fi hotspot isn't new: we've seen Comcast, Marriott, and others do it as well. For AT&T, the "trial" was meant to test whether inserted ads gave it a "secure and fast" way to offer free Wi-Fi service. "Our industry is constantly looking to strike a balance between the experience and economics of free Wi-Fi," AT&T tells Recode. At a minimum, inserting ads without warning users ahead of time isn't a great practice.