In recent years, T-Mobile has been whitelisting some apps that customers can use and they won't touch their monthly data allotment. This started with Music Freedom where a number of streaming music apps were whitelisted, but T-Mobile quietly whitelisted a number of speed test apps (like SpeedTest.net, OpenSignal and others). Now this was so that users could check the speed of the network without using their data. This benefitted T-Mobile as they were able to get more users to test the speed and they could use that data to boast about their network. But now it appears that a high schooler has figured out a way to use this to get free data on the magenta network.
A high schooler named Jacob Ajit, had a T-Mobile prepaid SIM card which had no active service on it, but found out a way to use T-Mobile's network nonetheless. Ajit does explain in his full write up of his experiment (which is linked in the sources below) that he noticed that T-Mobile's website would load up, and then SpeedTest.net would also load up. So he figured that T-Mobile was simply looking for a formatted "/speedtest" folder. And if it was found, then the request would go through. So the teen put a /speedtest folder on his website, and then loaded the site with some other files and accessed it without a problem. He even used a proxy server to access the same web site again, and it was still successful.
It's a loophole, but not one that is really going to affect any users. Unlike others, that could affect the security of accounts or smartphones. But it is a loophole that T-Mobile would likely want closed. It's also not something that every user is going to be able to use and get free data from T-Mobile. Since it would need to have that folder on every website for it to work. Ajit did say that he's reached out to T-Mobile and informed them of this loophole. So far, T-Mobile hasn't made any comment about the loophole just yet, but that could change in the very near future.