Mayer testified in front of the Senate Commerce Committee Wednesday regarding two massive data breaches at Yahoo.
"Unfortunately, while all our measures helped Yahoo successfully defend against the barrage of attacks by both private and state-sponsored hackers, Russian agents intruded on our systems and stole our users' data", she said.
For Yahoo, lawmakers are probing a 2013 breach, which the company reported in December of 2016 as it proceeded with its plans to merge with Verizon. That means the perpetrators had three years to dive into Yahoo user accounts, and to any other internet accounts registered with the same usernames and passwords.
A representative for Mayer said on Tuesday she was appearing voluntarily. "It's going to take an attitude change among companies such as yours that we've got to go to extreme limits to protect our customers' privacy". Roger Wicker, R-Miss., asked all of those testifying, including the interim and former CEOs of Equifax, Paulino de Rego Barros Jr. and Richard Smith, as well as Entrust Datacard CEO Todd Wilkinson, if they took issue with Nelson's contention that a "mere company" can not withstand persistent attacks from state-backed hackers without the help of the National Security Agency.
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A breach in 2014 affected 500 million Yahoo accounts and, in a first, led to the U.S. government criminally charging two Russian spies for cyber crimes.
Senators questioning the CEOs were not optimistic about future security breaches. Bill Nelson (D-Fla.) said, "but when".
"We now know that Russian intelligence officers and state-sponsored hackers were responsible for highly complex and sophisticated attacks on Yahoo's systems", Mayer said.
Thune told reporters after the hearing the Equifax data breach had created "additional momentum" for Congress to approve legislation.
Mayer volunteered to testify on data breaches, but only after being subpoenaed.