"At Yahoo, we have a deep understanding of the threats facing our users and continuously strive to stay ahead of these threats to keep our users and our platforms secure," she said. The breach disclosed last week is the latest black eye for Ms. Mayer, whose failed turnaround effort resulted in Yahoo's agreement in July to sell its core operations to Verizon for $4.8 billion. It is unclear whether the episode will affect the sale. Although Yahoo's email users are its most loyal and frequent customers, the company has been losing market share in email for years. "Yahoo is already suffering. I don't think they'll suffer more because of this," said Avivah Litan, a security analyst with the research firm Gartner.