T-Mobile's Latest Data Breach Linked to SIM Swap Attacks

Back in August, T-Mobile suffered a massive data breach impacting more than 50 million current, former, and prospective T-Mobile users, and now the cellular company is dealing with another smaller data breach incident.

Reports yesterday suggested that T-Mobile was aware of unauthorized activity affecting some customer accounts, and now, T-Mobile has confirmed that those reports were due to SIM swap attacks affecting a "very small number of customers."

In a statement to Bleeping Computer, T-Mobile said that impacted customers had been informed that they had been the victim of SIM swap attacks. In a SIM swap attack, social engineering is used to persuade T-Mobile employees to reassign the phone numbers linked to a person to someone else, allowing attackers to take over a phone number. This can be devastating, as phone numbers are often linked to email accounts, banking accounts, and other sensitive information.

We informed a very small number of customers that the SIM card assigned to a mobile number on their account may have been illegally reassigned or limited account information was viewed.

Unauthorized SIM swaps are unfortunately a common industry-wide occurrence, however this issue was quickly corrected by our team, using our in-place safeguards, and we proactively took additional protective measures on their behalf.

T-Mobile says that the attack has been mitigated and that the issue has now been corrected, but the company has not provided specific details on the number of customers impacted nor how the hackers were able to execute the SIM swap attacks.

In the August data breach, attackers were able to obtain phone numbers, addresses, birth dates, social security numbers, driver's license and ID info, IMEI numbers, and IMSI numbers for more than 50 million people, with the information offered up for sale.

T-Mobile CEO Mike Sievert apologized for the breach at the time, and said that T-Mobile was "truly sorry" for the incident, which was the result of a "bad actor" who used knowledge of T-Mobile's technical systems to gain access to testing environments, using brute force attacks to access T-Mobile's IT servers.

To prevent future attacks, T-Mobile entered into a long-term partnership with cybersecurity experts at Mandiant and with consulting firm KPMG LLP, and the company said that it was planning a multi-year investment to improve security.