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(CNN) - A breach of credit and debit card data at Target may have affected as many as 40 million people who shopped at the store in the three weeks after Thanksgiving, the retailer said Thursday.
The Secret Service, charged with safeguarding the nation's financial infrastructure and payment systems, confirmed it was investigating the breach last Wednesday.
Asked if companies are prepared to prevent such breaches, security expert Shawn Henry said, "I don't think you can prevent them."
"Right now the offense outpaces the defense, and the most sophisticated adversaries are going to get on the network," says Henry, president of CrowdStrike, and former FBI executive assistant director.
"The most companies can really do, is hope that they can detect this when it occurs, and they do that by vigilantly hunting on their network," said Henry.