Yahoo shares fell almost 5% on Thursday after the technology company disclosed a second massive data breach that raised fears Verizon might kill a deal to buy its core Internet business.

Silicon Valley-based Yahoo said late on Wednesday that it had uncovered a 2013 cyber attack that compromised data of more than 1 billion user accounts, the largest breach in history.

That followed Yahoo’s disclosure in September of a separate breach that affected over 500 million accounts, which the company said it believed was launched by different hackers.

Yahoo  YHOO -6.11%  shares were down 4.8% at $38.93 in midday trade as the latest revelation cast new doubt on whether Verizon Communications would proceed with a $4.83 billion agreement to buy Yahoo’s core Internet business, struck in July.

Verizon is now seeking to persuade Yahoo to amend the terms of the acquisition agreement to reflect the economic impact of the data breaches, according to people familiar with the matter.

Source: Here’s How Much Yahoo Shares Are Dropping After Latest Hack Reveal