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A Sonic Corp. Restaurant Ahead Of Earnings Figures
Signage stands outside a Sonic Corp. drive-in fast food restaurant in Knoxville, Tennessee, U.S., on Saturday, March 25, 2017.
Bloomberg—Bloomberg via Getty Images

Millions of credit card numbers may have been stolen in a security breach at the fast food chain Sonic Drive-In.

Sonic confirmed the breach with KrebsOnSecurity, a security news website, which found that about five million credit and debit card accounts were put up for sale by hackers on Sept. 18. Krebs reports that many of the cards in the for-sale batch had recently been used at Sonic locations.

Sonic said it was investigating the breach at some of the fast food chain's locations. Sonic did not immediately respond to Money's request for comment.

"Our credit card processor informed us last week of unusual activity regarding credit cards used at SONIC," the company said in a statement to Krebs. "The security of our guests’ information is very important to SONIC. We are working to understand the nature and scope of this issue, as we know how important this is to our guests. We immediately engaged third-party forensic experts and law enforcement when we heard from our processor. While law enforcement limits the information we can share, we will communicate additional information as we are able."

It is not clear if all the stolen credit card numbers were linked to Sonic. According to Krebs, it is possible that other companies were affected by the breach.

Check if your information was compromised in the breach by keeping up to date on your online bank statements and credit card statements. You can also request a credit report from your credit bureau to see if there are any irregularities.