It happened to shoppers who used a credit or debit card at Target in the day before Thanksgiving to just a few days ago.
While there is no link between customers who's information was leaked and current fraudulent activity on their bank accounts, as many as 40 million people's information could have been exposed to thieves.
Monica Combs shopped at Target during the time when the breach happened. She just learned her card was also compromised.
"I went on and was just checking my account and realized that there was 100 dollar transaction at Starbucks and I hadn't spent 100 dollars at Starbucks," she says.
Monica Combs is just one of millions who've been at Target in recent weeks.
"I shop at Target, usually weekly," she continued.
The mega-store announced a major fraud to 40 million customers.
Credit and debit card account had been compromised. It could be one of the largest security breaches in history.
"If you have shopped at Target in the last month, you should immediately go online and start going through your online banking statements. Whether that's your credit card, your checking account, what ever account you may have used, start immediately combing through all of those and make sure all of the charges are ones that you've made," says Chelsea Maguire, Communications Director with the Better Business Bureau.
She knows just what you should look for to see if there's a problem.
"Look for any charges that are out of the area that you were not there. Any stores you didn't shop at whether they're tiny charges or large charges," she added.
The stolen data included names, card numbers, expiration dates and even the embedded code on the magnetic strip.
Monica was able to call her bank and get her money back. And despite being a victim of fraud, it won't change her habits.
"I don't want to stop shopping, I mean, there's so many things that could put you at risk for security stuff in general. Daily. So it's just a little thing," she says.
Target won't say how the breach happened, but says the problem is fixed.
It doesn't appear online sales are affected.
If you suspect your information is compromised contact your bank and law-enforcement.