Yahoo has revealed that at least 500 million user accounts were stolen from its network in 2014. The hijacked information includes names, addresses, phone numbers, dates of birth and encrypted passwords. However, payment card data, bank account information and unprotected passwords have not appeared to be compromised.

According to popular cryptologist Bruce Schneier, the data breach is the biggest ever. He adds that it is too early to determine the hack's impact on Yahoo and its millions of users.

Three US intelligence officials believe that the attack is state-sponsored judging from the incident's resemblance to the previous hacks that were traced to Russian intelligence agencies or their affiliated hackers.

However, it remains unclear how Yahoo's disclosure will affect its deal with Verizon. In July, the New York-based communications company has agreed to buy Yahoo's core internet properties for $4.83 billion. A recent statement has Verizon announcing that it was informed of the breach only within the last two days.

A report about Yahoo's data hack has been initially unveiled on technology domain Recode. It came after an August narrative has surfaced on the Motherboard. The story on the technology news site focused on a cyber criminal known as Peace who was selling the data of around 200 million Yahoo users. Although the authenticity was not confirmed, the hacker has claimed responsibility. Previously, Peace has also attempted to sell millions of accounts at MySpace and LinkedIn on a hacker forum.

Originally, Yahoo has stated that it was investigating the situation. A couple of months later, the breach has turned out to be even worse. The company is now working with law enforcement units. An FBI spokesperson shares that the agency is aware of the intrusion and is investigating the matter.

US Senator Richard Blumenthal has called for tougher legislation to make sure companies are properly and promptly notifying consumers when their data have been compromised.

Meanwhile, Yahoo has urged users to change their passwords if they haven't since 2014. The company has one billion active users for all its internet services which include finance, online shopping and fantasy football. In June, Yahoo has told CNET that its mail service alone has 225 million active monthly users.