T-Mobile has revealed a second data breach that occurred in 2023, which reportedly exposed customer data and account PINs, leaving many T-Mobile users vulnerable to potential fraud and identity theft.

“In March 2023, the measures we have in place to alert us to unauthorized activity worked as designed and we were able to determine that a bad actor gained access to limited information from a small number of T-Mobile accounts between late February and March 2023,” T-Mobile explained in a letter to customers affected by the breach.

“The information obtained for each customer varied, but may have included full name, contact information, account number and associated phone numbers, T-Mobile account PIN, social security number, government ID, date of birth, balance due, internal codes that T-Mobile uses to service customer accounts, and the number of lines,” the company added in the letter.

T-Mobile already disclosed a data breach this year, on January 19.

According to T-Mobile, the hackers were able to access the data by exploiting a vulnerable API. Unfortunately, data breaches are almost the norm for T-Mobile; the company has disclosed several other breaches in recent years, including the August 2021 breach which affected 49 million customers.

“The fact that the attackers were able to operate on the T-Mobile network undetected for a month, stealing sensitive customer information without anyone’s knowledge is very concerning. Given that victims were unaware their data had been compromised, they would not have been on guard for phishing scams or been monitoring their accounts for fraudulent transactions, so it is likely attackers would have been able to exploit the stolen data during this time, completely under the radar.”