NBC News
Ben Popken and Jo Ling Kent

After staying mostly out of the public eye since the sale of Yahoo to Verizon, former Yahoo CEO Marissa Mayer emerged to take literal center stage at a very public drubbing on her former company's mega data breaches.

Lawmakers on the Senate Commerce Committee grilled Mayer all day Wednesday, along with Equifax's former and interim CEOs, and an executive from Yahoo's new owner, Oath — a subsidiary of Verizon — over what responsibility they and their companies had for the historic data breaches that saw billions of Americans' privacy compromised.  

"I want to apologize to each and every one of our users," Mayer told the Senate panel, apologizing for two hacks that targeted a staggering 3 billion users. One of the hacks was committed by Russian operatives, the U.S. says. Several Russian agents were arrested by the Department of Justice for their involvement.

And it could happen again.

"I believe that all companies — even the most well-defended ones — could fall victim to these crimes," said Mayer, noting that all companies are under threat from persistent hack attempts and need the help of the U.S. government and law enforcement to go after and arrest the hackers responsible.

Equifax and Yahoo both testified that they had increased security measures and heightened defenses since the attacks. Mayer said customers had been forced to change passwords and that steps were taken to tighten data security.

"Are customers more secure today than before the breach?" Kansas Senator Jerry Moran asked.

Karen Zacharia, Verizon's deputy general counsel, opened her mouth and gestured without saying anything before testifying that the company "has always taken privacy very seriously."

Senators also blasted Equifax for the security lapses that led to loss of private financial history for 145.5 million Americans.

"Is Equifax really the victim?" asked Senator Bill Nelson of Florida.

"We were the victim of a criminal attack," said former Equifax CEO Rick Smith in his fifth appearance in a Congressional hearing on the breach.

"Equifax is not the victim, " said Nelson. "It's the poor customers of Equifax."

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