Verizon's general counsel Craig Silliman told reporters the data breach could trigger a clause in the deal that would allow the U.S. wireless service company to not complete it.
"I think we have a reasonable basis to believe right now that the impact is material and we're looking to Yahoo to demonstrate to us the full impact. If they believe that it's not then they'll need to show us that," he said, declining to comment on whether talks are under way to renegotiate the purchase price.
A yahoo spokesman said: We are confident in Yahoo’s value and we continue to work towards integration with Verizon."
The deal has a clause that says Verizon can withdraw if a new event "reasonably can be expected to have a material adverse effect on the business, assets, properties, results of operation or financial condition of the business.”
Silliman said the U.S. Federal Trade Commission has approved Verizon's acquisition of Yahoo, but it still needs approval from the European Commission and the U.S. Securities and Exchange Commission is reviewing the proxy.
Verizon has had preliminary briefings from Yahoo but it still needs "significant information" from Yahoo before it makes a final decision on the materiality of the hacking of at least 500 million email accounts, Silliman said.
He said Verizon is "absolutely evaluating (the breach) and will make determinations about whether and how to move forward with the deal based on our evaluation of the materiality."