Last month tech firm Yahoo confirmed that at least 500 million user accounts were hacked back in 2014. User information was stolen during the 'state-sponsored' attack, but little else has been revealed about the attack. 

It's now feared that the number of account holders impacted could be significantly higher than first reported. Speaking to Business Insider, a former Yahoo employee says the type of breach that was reported would have gave access to a much larger group of user account information. The source says this would be the case due to the make-up of Yahoo's back-end system. 

"I believe it to be bigger than what’s being reported," the executive, who no longer works for the company but claims to be in frequent contact with employees still there, including those investigating the breach, told Business Insider. "How they came up with 500 is a mystery."

This source believes the number of accounts accessed and stolen could be anywhere between 1 billion and 3 billion. The company currently has 1 billion monthly active users around the world. The hack took place back in 2014, when the number of monthly active users was between 700 million and 1 billion.

Yahoo has not provided any further details about the hack, how it happened or when it was discovered. The company say it's an active investigation. The company has said they will contact any users whose accounts may have been compromised.