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: The simplest thing to do: Put a freeze on your credit. Here’s how it works. Whenever anyone applies for credit, the would-be lender pulls their credit report from one of the three bureaus, Equifax, Experian, or TransUnion. If you institute a security freeze at each of the three credit bureaus, nobody will be able to access your credit report, so identity thieves won’t be able to open any new accounts in your name — period. There are some downsides to this option. First, there’s the cost. The price of a security freeze varies by state — you can check yours here — but it’s typically $5 to $10 per credit agency. (It’s often free for people who have already been victims of identity theft.) More of a problem is that whenever you want to allow someone to check your credit, you’ll need to pay a fee to lift the freeze. And that may happen more often than you expect because your credit report gets pulled not just for credit applications but often when you sign up for a cell phone contract or apply for a new apartment or job as well. The credit agencies will give you a password to lift the freeze and charge up to $12 each time you do it — so this option can get pricey. Finally, if you’re afraid that an identity thief may already have used stolen information to open accounts in your name, a credit freeze won’t help. To find out if that’s the case, you’ll have to check your credit report, which you can do for free three times a year at annualcreditreport.com. If you suspect fraud, you’ll want to notify your financial institutions, change your passwords, watch your statements, and file a police report. Ready to act? You can place a credit freeze online, right now: Place a security freeze at Experian Place a security freeze at Equifax Place a security freeze at Transunion RELATED: Is my data safe? What should I do if I have been a victim of a data breach? What should I do if I have been the victim of identity theft? How can I protect myself from ID theft? Do I need identity theft insurance?