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Target CEO promises to make 'significant changes' after huge data breach

Gregg Steinhafel, CEO of the retailer currently dealing with the fallout from a massive security breach compromised that millions of customers' credit and debit accounts, pledges that Target will pay back "any possible fraudulent activity" on those accounts.

The head of retail powerhouse Target is pledging to "make significant changes" in the wake of the latest revelation that as many as 110 million customers were ensnared in a massive data heist at stores across the country.

"Clearly, we're accountable and we're responsible. But we're gonna come out at the end of this a better company," Gregg Steinhafel — Target chairman, president and chief executive officer — told CNBC's Becky Quick in an exclusive interview that will air in full on Monday morning. "And we're gonna make significant changes."

Customers can change their PIN or get a new card, says Gregg Steinhafel, Target chairman & CEO, explaining what consumers can do to address any lingering security concerns.

Steinhafel said that the embattled company will "get down to the bottom of this," adding: "We are not gonna rest until we understand what happened and how that happened." He also apologized for the sweeping theft, saying: “I’m personally very sorry that this whole event even happened.”

The retailer has said at least 70 to 110 million customers — a more massive number than previously disclosed in December, when the breach was first reported — were struck by the holiday-season data theft, making it one of the largest security breaches of its kind.

The stolen data includes names, mailing addresses, phone numbers or email addresses for up to 70 million individuals. 

When asked why it took the company four days to notify customers of the breach, Steinhafel said it “wanted to make sure our stores and our calls centers could be as prepared as possible,” adding that employees “worked around the clock to try and do the right thing.”

And Steinhafel told CNBC that the company still doesn't yet "know the full extent of what transpired."

Gregg Steinhafel, Target chairman & CEO discusses the company's response when it first learned of its massive security breach and how it worked with banks to notify and protect customers.

"But what we do know was there was malware installed on our point-of-sale registers. That much we've established," he said. "We removed that malware so that we could provide a safe and secure shopping environment."

He added that the company is in the midst of a criminal investigation — and "we can only share so much." 

While Target investigates how the breach happened, Steinhafel is pushing for new credit card technology for American consumers — a chip-and-PIN-number system called EMV technology that replaces vulnerable magnetic strips. However, switching the entire payment system will be a steep undertaking — and it likely won’t happen for shoppers in the U.S. until the end of 2015.

NBC News' Ben Popken contributed to this report.