Update: According to Tom's Guide, the incident affected approximately 400 T-Mobile customers. The attacker or attackers apparently hacked into T-Mobile's system with the intent of hijacking customer accounts, but the breach is over and the affected customers have been notified.

T-Mobile appears to have suffered another data breach, the company's third in less than a year.

Unlike the T-Mobile account breach disclosed in late December, this time because it involved "customers' full names, addresses, account numbers, social security numbers, customer account personal information numbers (PINs), account security questions and answers (and) dates of birth,

This is more serious.

This is according to canned letters obtained by Bleeping Computer that have been sent to T-Mobile customers whose accounts were compromised.

An unknown attacker apparently "used this information to port your line to another carrier without your permission," the February 9 letter adds.

"T-Mobile has identified this activity, stopped the unauthorized access, and taken steps to prevent a recurrence.

"Ported" or "SIM-swapped" numbers are serious enough, as they can be leveraged to hijack other accounts or steal cryptocurrency. However, information made public in such an obvious account breach can do more than steal phone numbers.

In many cases, all that is needed to completely steal someone else's identity is their full name, date of birth, social security number, and current address. These are all part of the T-Mobile data that was compromised in this case.

So far, there is no information on how many T-Mobile customers may have been affected. It is also not known whether the leaked accounts are the result of a massive data breach (as happened last March) or a series of individual account takeovers, such as those resulting from weak or reused passwords.

Tom's Guide has reached out to T-Mobile for answers to these questions and will update this article as we receive them.

T-Mobile customers who received a letter related to this most recent incident or series of incidents are entitled to two years of free credit monitoring and identity theft protection provided by TransUnion. They are also required to change their account PIN and account security questions and answers.

Tom's Guide urges affected T-Mobile customers to accept the company's offer of assistance and follow its advice to protect their accounts.

Concerned customers can call T-Mobile by dialing 611 from a T-Mobile cell phone or by dialing 1-800-937-8997 from any phone.