Yahoo confirmed that about half a billion user accounts have been stolen in 2014, blaming "state-sponsored" hackers in what appears to be the biggest cybersecurity breach in history.

Bob Lord, chief information security officer of Yahoo, said on Thursday that hackers breached into its network late in 2014. Stolen personal data included names, email addresses, telephone numbers, dates of birth, "hashed passwords and, in some cases, encrypted or unencrypted security questions and answers."

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He, however, said that other sensitive information including credit card data or bank account information were not affected, thus believing the attack was state sponsored, BBC reported.

Yahoo, which is selling its core business to Verizon Telecommunications Inc. in a $4.8 billion deal, believes that the perpetrators are no longer in the internet company's network.

The company is encouraging not only those affected users but all of its clients to promptly update their passwords and security questions and review their accounts for malicious activities, CNN Money reported.

According to The Wall Street Journal, Yahoo started investigating for possible theft in July after hackers claimed to sell 280 million Yahoo accounts. Although the company found that the sale was a hoax, it investigated further eventually finding out that it had been breached by a "state-sponsored actor."

Yahoo is cooperating with law enforcement to investigate about the breach.

"The FBI is aware of the intrusion and investigating the matter. We take these types of breaches very seriously and will determine how this occurred and who is responsible," an FBI spokesperson said.

The spokesperson further said that it will cooperate with the private sector and share the information so they can protect their systems from cyber criminals.

TagsYahoo, Cyberattack, cybersecurity, personal data theft, hackers