"It's shocking that a ... beloved lawyer took the hit for the CEO given all the departments involved," said the former executive, who spoke on condition of anonymity.
Communications, mail, engineering and legal reported directly to Mayer, the former executive said. Indeed, Yahoo Mail was Mayer's "one big product focus," the former executive said. "How is she not responsible?"
Yahoo disclosed two separate data breaches last year, both among the biggest in history. A 2013 attack revealed in December affected more than 1 billion user accounts. In a separate 2014 attack, disclosed in September, information was stolen from at least 500 million user accounts.
Mayer said she worked with various teams to disclose the hacks to users and government officials.
"However, I am the CEO of the company and since this incident happened during my tenure, I have agreed to forgo my annual bonus and my annual equity grant this year and have expressed my desire that my bonus be redistributed to our company's hardworking employees, who contributed so much to Yahoo's success in 2016," she said a blog post.
Yahoo has vowed its commitment to cybersecurity. "With security protocols and password changes in place, approximately 90 percent of our daily active users have already taken or do not need to take remedial action to protect their accounts, and we're aggressively continuing to drive this number up," Mayer said during a January earnings report.
— Reporting by CNBC's Sally Shin. Written by Anita Balakrishnan.