Usernames and passwords of some of Yahoo’s email customers have been stolen and used to gather personal information about people those Yahoo mail users have recently corresponded with, the company said Thursday.

Yahoo didn’t say how many accounts have been affected. Yahoo is the second-largest email service worldwide, after Google’s Gmail, according to the research firm comScore. There are 273 million Yahoo mail accounts worldwide, including 81 million in the U.S.

It’s the latest in a string of security breaches that have allowed hackers to nab personal information using software that analysts say is ever more sophisticated. Up to 70 million customers of Target stores had their personal information and credit and debit card numbers compromised late last year, and Neiman Marcus was the victim of a similar breach in December.

“It’s an old trend, but it’s much more exaggerated now because the programs the bad guys use are much more sophisticated now,” says Avivah Litan, a security analyst at the technology research firm Gartner. “We’re clearly under attack.”

Yahoo email account passwords stolen , 1/30/14

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