Until last week, a bug on a T-Mobile website let hackers access personal data such as email address, a customer’s T-Mobile account number, and the phone’s IMSI, a standardized unique number that identifies subscribers. On Friday, a day after Motherboard asked T-Mobile about the issue, the company fixed the bug.

The flaw, which was discovered by security researcher Karan Saini, allowed malicious hackers who knew—or guessed—your phone number to obtain data that could’ve been used for social engineering attacks, or perhaps even to hijack victim’s numbers.

“T-Mobile has 76 million customers, and an attacker could have ran a script to scrape the data (email, name, billing account number, IMSI number, other numbers under the same account which are usually family members) from all 76 million of these customers to create a searchable database with accurate and up-to-date information of all users,” Saini, who is the founder of startup Secure7, told Motherboard in an online chat. (T-Mobile said that, in fact, the company has 70 million customers, not 76).

“That would effectively be classified as a very critical data breach, making every T-Mobile cell phone owner a victim,” he added.
[…]
Karsten Nohl, a cybersecurity researcher who has done work studying cellphone security, told Motherboard that, theoretically, by knowing someone’s IMSI number, hackers or criminals could track a victim’s locations, intercept calls and SMS, or conduct fraud by taking advantage of flaws in the SS7 network, a backbone communications network that is notoriously insecure. Still, Nohl added that “there is no obvious way to make money easily with just an IMSI,” so it’s hard to tell whether such an attack would be attractive to cybercriminals.
[…]
a blackhat hacker who asked to remain anonymous warned Motherboard that the recently patched bug had been found and exploited by other malicious hackers in the last few weeks.

“A bunch of sim swapping skids had the [vulnerability] and used it for quite a while,” the hacker told me, referring to the criminal practice of taking over phone numbers by requesting new SIM cards impersonating the legitimate owners by socially engineering support technicians.

To prove their claim, the hacker sent me my own account’s data.

Source: T-Mobile Website Allowed Hackers to Access Your Account Data With Just Your Phone Number

On the positive side, T-Mobile gave the discoverer a bug bounty and tried to close the hole with an update. On the negative side, their patch didn’t close the hole.