McDonald’s client base was hacked

Hackers accessed a database of McDonald’s Corp that contained the email addresses and birth dates of people who voluntarily signed up for its websites and promotions.

The company immediately tried to calm media by saying the security breach did not include any Social Security numbers, credit card accounts or sensitive financial information.

The hacked data was on computer systems operated by an email database management firm hired by McDonald's long-time business partner Arc Worldwide, the marketing services arm of advertising firm Leo Burnett, McDonald's spokeswoman Danya Proud said in a statement.

"The incident has resulted in an investigation by law enforcement authorities. Arc and McDonald's are cooperating with the appropriate authorities," Proud said.

"We are also working with Arc and their database management firm to understand how the security was bypassed," she said.