The company says it believes the usernames and passwords weren’t collected from its own systems, but from a third-party database.

NEW YORK — Yahoo said Thursday that usernames and passwords of its email customers have been stolen and used to access accounts, but the company isn’t saying 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.

Yahoo Inc. said in a blog post on its breach that “The information sought in the attack seems to be names and email addresses from the affected accounts’ most recent sent emails.”
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