McDonald's is working with law enforcement authorities after malicious hackers broke into another company's databases and stole information about an undetermined number of the fast food chain's customers.McDonald's has also alerted potentially affected customers via e-mail and through a message on its website."We have been informed by one of our long-time business partners, Arc Worldwide, that limited customer information collected in connection with certain McDonald\u2019s websites and promotions was obtained by an unauthorized third party," a McDonald's spokeswoman said via e-mail on Saturday.McDonald's hired Arc to develop and coordinate the distribution of promotional e-mail messages, and Arc in turn relied on an unidentified e-mail company to manage the customer information database. This e-mail company's systems were hacked into.The data, which customers had provided voluntarily, doesn't include Social Security Numbers, credit card numbers, nor any sensitive financial information, she said."Rather, the limited information includes what was required to confirm the customer\u2019s age, methods to contact the customer, and other general preference information," the spokeswoman added.This means that customer data likely includes full names, phone numbers, postal addresses and e-mail addresses. The spokeswoman didn't say what information was required for age confirmation, so it's not clear if customers simply checked a box saying they were adults or if they had to provide their date of birth."In the event that you are contacted by someone claiming to be from McDonald's asking for personal or financial information, do not respond and instead immediately contact us," reads the McDonald's note to customers. The number to call is 1-800-244-6227.In addition to working with law enforcement agencies, McDonald's is probing the security breakdown at the company hired by Arc, which is the marketing services division of ad agency Leo Burnett. Arc's specialities include digital communications, direct marketing, promotions and shopper marketing, according to its website.The spokeswoman didn't say how many people are potentially affected and in what countries, besides the U.S. She also didn't say when the breach happened.