Dairy Queen announced on Thursday that the "Backoff" point-of-sale malware has infected at least 395 stores. `Payment card data was stolen.

The company has 4,500 independently owned businesses in the U.S. It believes that the "malware has been contained." The breaches occured between three weeks and a month in early August, according to their list.

CEO and President John Gainor wrote: “We deeply regret any inconvenience this incident may cause."

Dairy Queen said its investigation showed that a third-party vendor’s account credentials were used to access the systems at the affected locations.

The company said that customers names, payment card numbers and card expiration dates were stolen. However, no customer information such as Social Security numbers, PIN's or email addresses were stolen.

Backoff, which first appeared around October 2013, collects payment card data from a computer’s RAM where it briefly sits unencrypted after a card is swiped.