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The company says "malicious software planted by an outside source" may have been able to access the names, addresses and credit card numbers of customers who shopped at Qbookshop.com, Qbookshop.net, Motorbooks.com or WalterFoster.com between April 29, 2013 and June 17, 2013.
"Upon becoming aware of the incident, we took immediate steps to contain the breach and initiated an investigation," Quayside director of operations Joseph Cella wrote in the notification letter [PDF file]. "Our shopping cart is temporarily suspended until we are absolutely certain that all malware has been removed from our server and we have simultaneously initiated additional security features."
All customers who made purchases between April 29 and June 17 are being advised to place a fraud alert on their credit files.