If your company has experienced a data breach, it’s probably a good idea to thoroughly investigate it promptly.

Unfortunately, Yahoo didn’t, according to a new internal investigation. The internet pioneer, which reported a massive data breach involving 500 million user accounts in September, actually knew an intrusion had occurred back in 2014, but allegedly botched its response.

The findings were made in a Yahoo securities exchange filing on Wednesday that offered more details about the 2014 breach, which the company has blamed on a state-sponsored hacker.

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