Yahoo Inc. is evaluating whether an unidentified hacker has access to its user account data, following a 2014 hack that resulted in the theft of more than 500 million user account records.

In a regulatory filing Wednesday, Yahoo said law-enforcement authorities on Monday "began sharing certain data that they indicated was provided by a hacker who claimed the information was Yahoo user account data." Yahoo said it would "analyze and investigate the hacker's claim."

A company spokesman declined to elaborate, adding, "We did just receive the file and haven't yet completed the analysis."

The data could shed some light on what may be the largest theft of consumer data ever. Yahoo has said previously that it believes its networks were compromised in late 2014 by "state-sponsored" hackers who stole names, email addresses, telephone numbers and dates of birth of more than 500 million users. But information-security firm InfoArmor Inc. later said the data had been stolen by criminals, rather than a state-sponsored group.

An InfoArmor spokesman said Wednesday the company had shared its data with law enforcement.

In the Wednesday filing, Yahoo also said it is investigating whether the state-sponsored actor responsible for the hack created special files, called "cookies," that would have allowed it to bypass Yahoo's security and "access certain users' accounts or account information."

The company is facing 23 class-action lawsuits following the hack, the filing said.

Last month Verizon Communications Inc. Chief Executive Lowell McAdam said that his company is evaluating whether the hack will have a material impact on Yahoo's business, raising the possibility that Verizon might renegotiate its $4.8 billion deal to acquire Yahoo's core business.

A spokesman at the FBI declined to comment.

Write to Robert McMillan at Robert.Mcmillan@wsj.com

(END) Dow Jones Newswires

November 09, 2016 19:05 ET (00:05 GMT)

Copyright (c) 2016 Dow Jones & Company, Inc.
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