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A customer exits a T-Mobile store in Glendale, California, on August 1, 2014.
A customer exits a T-Mobile store in Glendale, California, on August 1, 2014.
Robyn Beck—AFP/Getty Images

It's happened again: If you're you applied for T-Mobile service or device financing anytime between Sept. 1, 2013, and Sept. 16, 2015, you've been hacked.

T-Mobile CEO John Legere announced yesterday that hackers breached Experian, the credit bureau that processes T-Mobile's credit checks. The fraudsters got names, addresses, birth dates, and — most unfortunately — Social Security numbers for some 15 million T-Mobile credit applicants. That includes both customers and people who submitted to a T-Mobile credit check but either cancelled or never activated their T-Mobile service.

While hackers did not access credit or debit card numbers during this breach, losing your Social Security number is actually worse. If an identity thief uses your credit or debit card, you can report fraudulent purchases when you see your bill. You should get your money back, and your financial institution will simply send you a new card. But with your Social Security number, an identity thief can open new accounts, run up debt in your name, and ruin your credit score—potentially before you even know that a thief has your information. And you can almost never change your Social Security number.

Fortunately, there's one foolproof thing you can do to prevent this kind of identity theft: Freeze your credit report. Here's what we told victims of data breaches last year: