The CEO of T-Mobile got in a fight with a rights group over claims his company is throttling customers' data

T-Mobile is coming under fire online after CEO John Legere attacked civil liberties group EFF (Electronic Frontier Foundation) in a foul-mouthed rant.

The row comes after the EFF accused T-Mobile of violating Net Neutrality - the principle that all data must be treated equally.

"Who the f--- are you, anyway, EFF?" Legere asked. "Why are you stirring up so much trouble, and who pays you?"

In response, many internet users have supported the group and attacked T-Mobile, using the hashtag #WeAreEFF.

The heart of the issue is "Binge On," a new service T-Mobile offers. It lets video data from certain companies not count towards users' monthly data cap, leaving them free to watch much more video content. But according to tests run by the EFF, it throttles all video content it can detect to the speed of 1.5Mps, "even when the phone is capable of downloading at higher speeds, and regardless of whether or not the video provider enrolled in Binge On."

Combined with the fact it "zero rates" some data, meaning it doesn't contribute to user limits, this is arguably a violation of net neutrality, which stipulates that carriers cannot discriminate against or preferentially treat any types of data - it all has to be treated equally. Net Neutrality is now part and parcel of FCC regulations in the US, so if T-Mobile is violating them, it could face consequences.

T-Mobile is already due to meet with the FCC to discuss zero rating, and there's some debate as to whether or not the company's actions technically violate net neutrality.

But in responding to recent criticisms, John Legere has raised the ire of many internet activists. On Thursday, he published a video on YouTube claiming to "set the record straight" about Binge On, framing it as about "customer choice."

The CEO then followed it up with a Q&A session on Twitter, in which he recorded a selfie-message targeting the EFF.

EFF asked: "Does Binge On alter the video stream in any way, or just limit its bandwidth?"

To which Legere responded: "So what Binge On does, it includes a proprietaty technology and what the technology does is not only detect the video stream but also select the bitrate to optimise to the mobile device. That's part A of my answer. Part B of my answer is: Who the f--- are you, anyway, EFF? Why are you stirring up so much trouble, and who pays you?"

People immediately questioned the wisdom of pitting a mobile carrier against a very well-regarded civil liberties group with decades of activism under its belt:

Twitter users subsequently rallied in support of EFF, using #WeAreEFF as a banner.

Legere has since struck a reconcilliatory tone - acknowledging that the EFF does "a lot of great things for a lot of consumers," but that "innovation can be controversial!"