To protect users, the company has forced password resets and invalidated the forged cookies.
Nevertheless, after the hacking incidents, Yahoo agreed to take off US$350 million from Verizon Communications' original offer to buy the internet company. The deal is expected to close this second quarter.
Because of the breaches, Yahoo said the company is facing about 43 class action lawsuits.
In a separate post, Yahoo CEO Marissa Mayer said she'll forgo taking her annual bonus because the 2014 breach happened on her watch.
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