Target's data-security nightmare threatens to drive off holiday shoppers during the company's busiest time of year. Shoppers leave a Target store in North Olmsted, Ohio Thursday, Dec. 19, 2013. Target says that about 40 million credit and debit card accounts may have been affected by a data breach that occurred just as the holiday shopping season shifted into high gear. Shoppers arrive at a Target store in Los Angeles on Thursday, Dec.