On Sept. 22, Yahoo announced that hackers had stolen data related to 500 million Yahoo users. The stolen data included email addresses, telephone numbers, dates of birth, encrypted passwords and security questions. The data could be used to link users to their banks, financial services, sensitive personal information and their social network. This was by far the largest consumer-data cyberbreach ever recorded.

But the stock market seemed to yawn in response. On Sept. 23, the first trading day after news of the breach reached...