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Computerworld - Security experts today warned users to be on the watch for targeted email attacks after a breach at a major marketing firm that may have put millions of addresses in the hands of hackers and scammers.

The addresses will also be invaluable to attackers playing in the high-stakes game of hacking major corporations like the one that RSA Security disclosed last month, a researcher added.

Last week, Irving, Texas-based Epsilon admitted that names and email addresses of a "subset of Epsilon clients" were accessed by hackers. Epsilon, which sent 6.5 billion messages in 2009, runs email marketing and customer loyalty campaigns for some of the country's biggest banks, credit card companies and retailers, including American Express, Best Buy, Citibank, Capital One, Kroger, Visa and U.S. Bank.

Those companies and others have acknowledged the Epsilon hack, and warned their customers to be wary of spam, according to a list compiled by security blogger Brian Krebs.

Experts today said that scammers will probably put the email addresses to work in targeted attacks, often dubbed "spear phishing," that try to dupe users into divulging their log-on credentials.

Spear phishing is most commonly used by identity thieves hoping to obtain access to consumers' and businesses' bank or credit card accounts, although the term is also used to describe any attack aimed at specific individuals rather than relying on huge volumes of messages.

"It will be no surprise if the addresses are used for targeted attacks, whether spear phishing or to deliver malicious links to users," said Graham Cluley, a senior technology consultant with U.K.-based security company Sophos.

Recipients unaware of the Epsilon hack will be more likely to click on such links or open malware-infected attachments because the incoming messages are from a company with which they have an established relationship, said Cluley.

HD Moore, the chief security officer at Rapid7, echoed Cluley. "People already expect to get messages from these companies," Moore said.

Cluley thought that the danger might be greater in the future, after the news of the Epsilon breach has quieted. "This is in the news now, but the email addresses could be exploited in 6 or 12 months, long after most people have forgotten about the incident," said Cluley.