Bank of America has hired an army of external contractors to help it patch software vulnerabilities exposed by advanced artificial intelligence models including Anthropic's Mythos, the bank's CEO, Brian Moynihan, told CNBC on Wednesday.

In an interview after the bank's third-quarter earnings release, Moynihan said that earlier this year the company rushed to bolster a wide spectrum of its internal and vendor code using thousands of employees and outside personnel.

"That risk is real risk, and that's why when the frontier-type models came out. … A lot of us sat down and said, 'How do we figure out how to get through this quickly?" Moynihan told CNBC's Leslie Picker.

"We put on 1,000 contractors to help us do the patching work in the second quarter, and those people remain here today, helping us, on top of the 3,000 people we have in that area," he said.

The bank spent a "couple hundred million plus" on the software remediation and regulatory issues related to anti-money laundering compliance in the third quarter, Moynihan told analysts earlier Wednesday.

The comments are some of the first by a major U.S. corporation to specify the manpower and dollars spent to protect software systems from Mythos, which was released to a small group of banks, technology firms and government agencies in April.

They also highlight the dual-edged nature of AI: While it can help companies save money by boosting productivity, the ability of models like Mythos to uncover and exploit decades-old software flaws has also forced firms into a costly defensive scramble.

"We've got to stay ahead of them, and we've got to be mindful that the people doing the software development are the people trying to sell us the services," Moynihan said.

Still, Moynihan — speaking as his company's shares slumped as much as 8% Wednesday — sought to reassure investors that the spending increase was temporary.

"Once you patch, you don't have to patch as much. … The pipeline moves through," Moynihan said, adding that the bank expects remediation costs to decline over time.

In other places, AI is helping to reduce expenses. Bank of America has about 130 internal AI projects, and savings from those efforts are expected to rise from $300 million this year to $700 million in 2027, Moynihan told analysts.

Bank of America's head count fell by about 3,400 employees in the past year as the firm cut roughly 5% of managers, he said. It's unclear how much of that workforce reduction is attributable to AI.