Yahoo announced they discovered attempts to access Yahoo mail accounts [1]. Not a huge amount of information has currently been released about what happened, but the usernames and passwords have come from some unnamed third party source, not from Yahoo.
Yahoo, and we at the Storm Center, advises you change your Yahoo email account passwords.
I’d also recommend being wary of any odd email messages from friends with Yahoo accounts that send you links or attachments in the next few days.
[1] http://yahoo.tumblr.com/post/75083532312/important-security-update-for-yahoo-mail-users
Chris Mohan --- Internet Storm Center Handler on Duty
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