Cyberattackers gained access to "a limited number" of Banner Health computer servers, including the servers that process payment card information where food and beverages are sold, according to an article on the Becker's Hospital Review website.
Up to 3.7 million people may be affected, making it the largest healthcare data breach of 2016.
Phoenix-based Banner said the attack targeted payment card data, and cards used at food and beverage outlets at certain Banner Health locations between June 23 and July 7.
The health system has returned to accepting all forms of payment at food outlets. Payment cards used to pay for medical services were not affected.
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