Your Social Security number and other sensitive credit information may have been compromised in last week’s massive data breach. A hacker, potentially an insider, put 3 billion data records of millions of Americans at risk at background check company National Public Data.

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While the breach occurred late last year, it has only just come to light. There is a way to check whether your data was exposed — and steps to take to protect it if it was, Fortune reports.

National Public Data has set up a website, npd.pentester.com, to check if your information is among the exposed accounts. To find out, all you need to do is to enter your first and last name, birth year, and the last four digits of your Social Security number.

National Public Data is advising people who have been affected to closely monitor their bank and other financial accounts. You can also subscribe to a credit monitoring service, such as LifeLock.

A third, and perhaps the most secure, way to proactively get ahead of any bad actors is to contact each of the three U.S. credit reporting agencies — Equifax, Experian and TransUnion — to put a freeze on your accounts. Doing this will ensure that a creditor will contact you first if anyone fraudulently tries to open an account in your name.

If you need to apply for credit during the freeze, you can temporarily lift it, and you can also set the timeframe for the freeze. The National Public Data website includes links to take you to the exact locations on the three credit reporting bureau websites where you can do this.

The U.S. Public Interest Research Group believes the credit freeze is the best protection against someone opening new credit accounts in your name.