The site found a batch of 5 million credit and debit card numbers for sale on a website.
Journalist Brian Krebs first reported the breach Tuesday.
KrebsOnSecurity reported that upon request for comment, Sonic confirmed that they were investigating a "potential incident" at some locations.
According to the statement, Sonic staff have called in forensic experts and law enforcement to help look into the issue. At this time, it's unknown how many locations are affected.
Sonic Drive-In burger.WikiMedia CommonsMillions of Sonic customers may have had their credit card information stolen. "While law enforcement limits the information we can share, we will communicate additional information as we are able".
Sonic has over 3,500 restaurants across the USA, including 17 in New Jersey, 15 in NY and four in CT.
"The security of our guests' information is very important to SONIC", wrote Christi Woodworth, Vice President of Public Relations. Cliff Hudson, the company's CEO, told Business Insider last week that the chain was attempting to invest in technology to better compete with both rival restaurant concepts and competitors such as Amazon.
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