Also see Verizon Is Said to See Yahoo Breach Affecting Acquisition (Bloomberg)
"The Securities and Exchange Commission’s guidance on cyberattacks says breaches can have a material impact and lead to lost revenue if proprietary information is stolen, or if companies “fail to retain or attract customers following an attack.”Verizon Says Yahoo Hack Could Reopen $4.8 Billion Deal Talks - The New York Times
“There are some serious questions about whether Yahoo’s customers will continue to use the service knowing that their data has been compromised,” said Jacob Olcott, vice president of BitSight Technologies, a security company. “That is likely to be the biggest question Verizon is trying to determine here. They acquired Yahoo, in part, because of its large customer base.”
Yahoo customers have been inquiring how to switch out email accounts after the breach. Earlier this month, the company turned off automatic email forwarding, making it harder for users to switch to alternative services."