Hackers roamed undetected in Equifax Inc.'s computer network for more than four months before its security team uncovered the massive data breach, the security firm FireEye Inc. said this week in a confidential note sent to some Equifax customers.
Equifax had previously disclosed that data belonging to approximately 143 million Americans was potentially accessed in May. FireEye's Mandiant group, which has been hired by Equifax to investigate the breach, said the first evidence of hackers' "interaction" with the company occurred on March 10, according to the client note, which was reviewed by The Wall Street Journal.
Equifax wasn't immediately available for comment.
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