Yahoo has admitted that some of its staff were aware of the 2014 data breach on its platform before its disclosure earlier this year. Yahoo suffered a massive data breach in 2014. It is believed that more than 500 million Yahoo Mail accounts were compromised in the incident.

In its recent Securities and Exchange filing, Yahoo stated, "The Company had identified that a state-sponsored actor had access to the Company's network in late 2014." The company has also set up an independent committee to investigate the matter. However, it let at least 18 months elapse before starting the investigation.

Yahoo disclosed the breach in late September this year. The company had entered into an acquisition deal worth $4.8 billion with Verizon around the same time. Verizon claims that it was informed of the breach only a few days before the public disclosure in September.

The new disclosure is expected to put further strain on the deal between the companies.

In response to Yahoo's latest announcement, Jim Gerace, Verizon's chief communication officer said, "We're still evaluating the situation and haven't come to any conclusions."

However, Yahoo maintains that breach did not materially affect its business. A Yahoo spokesperson said, "we continue to work towards integration with Verizon."

In its latest filing, Yahoo also revealed that hackers used computer coding called "cookies" to breaching its security system. The cookies let them view information in user accounts without requiring a password. Yahoo will also analyze information obtained by the FBI from a hacker.