CALIFORNIA (REUTERS) - The biggest known hack ever looks like it will peel off up to US$350 million (S$496.7 million) from the price Verizon is willing to pay for Yahoo.

A source has told Reuters that a deal revising the US$4.8 billion price tag is coming soon - all because of a hack that hit over a billion Yahoo user accounts.

Yahoo shares were up over a per cent on Wednesday (Feb 15). Verizon, on the other hand, slipped 0.7 per cent.

Verizon is looking to combine Yahoo's search, email, messenger and ad tools with the AOL unit it bought for US$4.4 billion in 2015 - part of its push for new sources of revenue in mobile video and advertising outside the overcrowded wireless market.

Yahoo has been under scrutiny by federal investigators and lawmakers since the reveal of the largest known data breach in history in December, mere months after it disclosed a separate major hack.

The US Securities and Exchange Commission has launched an investigation into whether Yahoo should have disclosed the breaches, which occurred in 2013 and 2014, sooner, according to a report in the Wall Street Journal last month.