Whoever penetrated Radisson's computers was able to remain there for more than half a year, during the period from November 2008 to May 2009.
Korallus said that Radisson is working with federal law enforcement authorities to investigate what happened.
"While the number of potentially affected hotels involved in this incident is limited, the data accessed may have included guest information such as the name printed on a guest's credit card or debit card, a credit or debit card number, and/or a card expiration date," he said in the letter.
A spokesperson for the company said that he couldn't provide more specific information. "[T]he forensic investigation is still underway, and we are unable to provide accurate estimates of the number of potentially exposed records at this time," the spokesperson said via e-mail. "As more information becomes available, and if disclosing it will not compromise the investigation, we will provide updates."
Korallus recommended that Radisson customers monitor their financial statements and credit reports for suspicious activity.
Radisson is offering those potentially affected with free credit monitoring for a year.
On Monday, the U.S. Department of Justice announced the indictment of Albert Gonzales, 28, of Miami, Fla., for allegedly hacking into corporate computers and stealing more than 130 million credit and debit cards. The DoJ called it the largest data breach indictment ever brought in the United States.
But such law enforcement successes appear to have done little to stem the tide of cybercrime. In the first half of 2009, the number of computer users affected by malware engineered to steal personal information increased by 600% compared to first half of 2008, according to PandaLabs.
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