The credit card numbers were put up for sale on the dark web.

A post on Krebs on Security's website says that there was an increase in fradulent charges on cards that had been used at Sonic Drive-In locations. They then traced it to a "shadowy underground network" where the cards were being sold.

The company says, "We are working to understand the nature and scope of this issue, as we know how important this is to our guests," a statement by the company said. "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's the same kind of breach that affected Home Depot and Target. The company is still investigating, and hasn't offered any kind of answers yet. They have 3,600 restaurants in 45 states.

If you've visited a Sonic, you're encouraged to keep an eye on your credit and transactions.