I can’t remember exactly what I was doing with my spare time as a 17-year-old teenager, but it sure wasn’t finding flaws in T-Mobile’s system. But I guess that’s why I had to actually put up with a data cap.

For Jacob Ajit, not so much of a problem. He discovered a flaw in T-Mobile’s filters that allowed him to use the network’s data, even when he hadn’t paid for anything.

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Ajit discovered that if he put a prepaid SIM with no credit into a phone, he was still able to connect to T-Mobile’s portal, where he could potentially buy more data. Then he noticed that his Speedtest app still worked, which meant that T-Mobile was allowing for images (what Speedtest uses to measure download speed) to be downloaded across its network without paying.

So, with a little messing around, Ajit worked out that anything in a folder labelled /speedtest was whitelisted by T-Mobile. He tried it out by loading up a music video on his server in a folder marked /speedtest, and sure enough, it worked.

To take the test to its logical conclusions, he set up a proxy server using the same rules, which he was able to use to freely browse the internet through T-Mobile’s network without ever paying for service.

Ajit alerted T-Mobile and disabled the proxy server before writing up his discovery on Medium, so don’t expect this to keep working. As Ajit says, it’s a “very simple” fix for T-Mobile to patch the hole.

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