Yahoo says all three billion of its accounts were hacked in a 2013 data theft, tripling its earlier estimate of the size of the largest breach in history, in a disclosure that lawyers say sharply increased the legal exposure of its new owner, Verizon Communications.

The news expands the likely number and claims of class action lawsuits by shareholders and Yahoo account holders, they said.

Yahoo, the early face of the internet for many in the world, already faced at least 41 consumer class-action lawsuits in US federal and state courts, according to company securities filing in May.

John Yanchunis, a lawyer representing some of the affected Yahoo users, said a federal judge who allowed the case to go forward still had asked for more information to justify his clients' claims.

"I think we have those facts now," he said.

"It's really mind-numbing when you think about it."

Yahoo said last December that data from more than one billion accounts was compromised in 2013, the largest of a series of thefts that forced Yahoo to cut the price of its assets in a sale to Verizon.

Yahoo on Tuesday said "recently obtained new intelligence" showed all user accounts had been affected.

The company said the investigation indicated the stolen information did not include passwords in clear text, payment card data or bank account information.

But the information was protected with outdated, easy-to-crack encryption, according to academic experts.

It also included security questions and backup email addresses, which could make it easier to break into other accounts held by the users.

Many Yahoo users have multiple accounts, so far fewer than three billion were affected but the theft ranks as the largest to date.

Verizon in February lowered its original offer by $US350 million for Yahoo assets after two massive cyber attacks at the internet company.