Here’s how to find out if your Social Security number was included in last week’s massive data breach

Last week’s staggering data breach at National Public Data put personal data, including the Social Security numbers, of billions of Americans in the hands of cybercriminals.

The breach occurred late last year, and the company acknowledged on its website last week that there were “potential leaks of certain data in April 2024 and summer 2024."

Data reaching back at least three decades is said to be included in the breach—and the inclusion of Social Security numbers has raised a number of concerns among people. Curious if you were affected? Here’s how to find out.

How can I find out if my Social Security number was part of the breach?

Pentester, a cybersecurity firm, has set up a tool to let you see if your data was part of the breach. Use a web browser to navigate to npd.pentester.com and enter your first and last name and birth year. You’ll see a list of breached accounts, including the last four digits of the leaked Social Security numbers.

What should I do if my Social Security number was included in the breach?

NPD is advising people who might have been affected to closely monitor their financial accounts. Most Americans don't keep close tabs on their checking and saving balances and don't examine every item on their credit card bills—and hackers count on that.

It’s also a good idea to set up credit monitoring to ensure no one is using your personal information—and contact one of the three U.S. credit reporting agencies ( Equifax , Experian , and TransUnion) to set up a fraud alert on your account, which will tell creditors to contact you before they open any new accounts or change your existing accounts. Once it’s set up with one agency, it will go into effect with the others. It remains active for one year and can be renewed.

If you're especially worried about identity theft, there's another option: A credit freeze , which prevents new credit from being issued without your direct permission.

"Your best protection against someone opening new credit accounts in your name is the security freeze (also known as the credit freeze), not the often-offered, under-achieving credit monitoring," notes the U.S. Public Interest Research Group .

This story was originally featured on Fortune.com