Marissa Mayer was given one final job for Yahoo: getting grilled by Congress. 
 
 Mayer, the former CEO of Yahoo, testified before a Senate Commerce Committee on Wednesday about a security breach that compromised three billion user accounts, making it the single largest breach in history. 
 
 "As CEO, these thefts occurred during my tenure and I want to sincerely apologize to each and every one of our users," Mayer said in opening remarks at the Senate hearing. 
 
 The breach took place in August, 2013 and was disclosed by Yahoo in 2016. Yahoo initially said it impacted more than one billion accounts. Verizon, which acquired Yahoo in June, revealed last month that the breach actually compromised all accounts. 
 
 At the data breach hearing, which also featured the current and former CEOs of Equifax (EFX), Mayer was pressed on why it took so long to disclose the breach and how it could have underestimated the impact by billions of accounts. 
 
 Related: Why Marissa Mayer couldn't save Yahoo 
 
 "Yahoo did not know of the intrusion in 2013. We learned of the intrusion by files presented to us in 2016," Mayer said. She deflected the increased estimate by noting it was revealed after her tenure at the company had ended. 
 
 Mayer left Yahoo when the acquisition closed in June. She walked away from the company with nearly $260 million at the time in stock and severance. 
 
 Sen. Brian Schatz, a Democrat from Hawaii, called it "unfathomable to the average person" that the CEOs of Yahoo or Equifax could walk away with "possibly a quarter of a billion dollars" despite the data security failures. 
 
 "You harm consumers and you walk away with the amount of money that a small city or county uses for their annual operating budget," Schatz said. 
 
 Sen. Jerry Moran, a Republican from Kansas, pressed the Yahoo and Equifax executives on whether consumers can expect their data to be safer today in the wake of the breaches. Verizon's chief privacy officer, Karen Zacharia, stumbled and stopped short of saying yes. 
 
 In her remarks, Mayer said companies are effectively in an "arms race" with private hackers and state-sponsored actors as their methods "become more sophisticated." 
 
 "The threat from state-sponsored attacks has changed the playing field so dramatically that today I believe all companies, even the most well defended ones, could fall victim to these crimes," Mayer said. 
 
 The Justice Department previously indicted two Russian spies for a separate 2014 breach of Yahoo's data. The source of the 2013 breach remains unknown. 

 WASHINGTON (Reuters) - Former Yahoo Chief Executive Marissa Mayer apologized on Wednesday for two massive data breaches at the internet company, blaming Russian agents for at least one of them, at a hearing on the growing number of cyber attacks on major U.S. companies. 
 
 “As CEO, these thefts occurred during my tenure, and I want to sincerely apologize to each and every one of our users,” she told the Senate Commerce Committee, testifying alongside the interim and former CEOs of Equifax Inc (EFX.N) and a senior Verizon Communications Inc (VZ.N) executive. 
 
 “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.” 
 
 Verizon, the largest U.S. wireless operator, acquired most of Yahoo Inc’s assets in June, the same month Mayer stepped down. Verizon disclosed last month that a 2013 Yahoo data breach affected all 3 billion of its accounts, compared with an estimate of more than 1 billion disclosed in December. 
 
 In March, federal prosecutors charged two Russian intelligence agents and two hackers with masterminding a 2014 theft of 500 million Yahoo accounts, the first time the U.S. government has criminally charged Russian spies for cyber crimes. 
 
 Those charges came amid controversy relating to alleged Kremlin-backed hacking of the 2016 U.S. presidential election and possible links between Russian figures and associates of President Donald Trump. Russia has denied trying to influence the U.S. election in any way. 
 
 Special Agent Jack Bennett of the FBI’s San Francisco Division said in March the 2013 breach was unrelated and that an investigation of the larger incident was continuing. Mayer later said under questioning that she did not know if Russians were responsible for the 2013 breach, but earlier spoke of state-sponsored attacks. 
 
 Senator John Thune, a Republican who chairs the Commerce Committee, asked Mayer on Wednesday why it took three years to identify the data breach or properly gauge its size. 
 
 Mayer said Yahoo has not been able to identify how the 2013 intrusion occurred and that the company did not learn of the incident until the U.S. government presented data to Yahoo in November 2016. She said even “robust” defenses are not enough to defend against state-sponsored attacks and compared the fight with hackers to an “arms race.” 
 
 Yahoo required users to change passwords and took new steps to make data more secure, Mayer said. 
 
 “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. She said “really aggressive” pursuit of hackers was needed to discourage the efforts, and that even the most well-defended companies “could fall victim to these crimes.” 
 
 The current and former chief executives of credit bureau Equifax, which disclosed in September that a data breach affected as many as 145.5 million U.S. consumers, said they did not know who was responsible for the attack. 
 
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 Senator Bill Nelson said “only stiffer enforcement and stringent penalties will help incentivize companies to properly safeguard consumer information.” 
 
 Thune told reporters after the hearing the Equifax data breach had created “additional momentum” for Congress to approve legislation. He said Mayer’s testimony was “important in shaping our future reactions.” 
 
 The Senate Commerce Committee took the unusual step of subpoenaing Mayer to testify on Oct. 25 after a representative for Mayer declined multiple requests for her voluntarily testimony. A representative for Mayer said on Tuesday she was appearing voluntarily.
– "I want to sincerely apologize to each and every one of our users,” Reuters quotes former Yahoo CEO Marissa Mayer as saying. Mayer testified in front of the Senate Commerce Committee Wednesday regarding two massive data breaches at Yahoo. In 2013, all 3 billion Yahoo accounts were hacked in what CNN reports was the largest data breach in history. Mayer says Yahoo, which originally said only 1 billion accounts were affected, didn't find out about the hack until it got data from the government in 2016 and still hasn't figured out how it happened, though she says Russian intelligence officers have launched attacks on Yahoo systems. A breach in 2014 affected 500 million Yahoo accounts and, in a first, led to the US government criminally charging two Russian spies for cyber crimes. One senator calls Wednesday's testimony, which also featured Equifax executives, "discouraging" because the companies don't appear to believe there's anything they can do to truly protect their users, CNBC reports. "The threat from state-sponsored attacks has changed the playing field so dramatically that today I believe all companies, even the most well defended ones, could fall victim to these crimes," Mayer says. Sen. Richard Blumenthal of Connecticut says enforcing punishments for data breaches on executives like Mayer could motivate companies to protect users' data. Mayer left Yahoo over the summer with nearly $260 million in stock and severance. "You harm consumers and you walk away with the amount of money that a small city or county uses for their annual operating budget," Sen. Brian Schatz of Hawaii says, calling it "unfathomable."