A hacker accessed more than 100 million credit card applications with US financial heavyweight Capital One, the firm said Monday, in one of the biggest data thefts to hit a financial services company.

FBI agents arrested Paige Thompson, 33, a former Seattle technology company software engineer, after she boasted about the data theft on the information sharing site GitHub, authorities said.

"The intrusion occurred through a misconfigured web application firewall that enabled access to the data," a statement by the US attorney's office in the northwest state of Washington said. 

"On July 17, 2019, a GitHub user who saw the post alerted Capital One to the possibility it had suffered a data theft."

It said the Virginia-based bank that specializes in credit cards contacted the FBI after confirming the data theft, which took place between March 12 and July 17 of this year.

"According to Capital One, the data includes data regarding large numbers of [credit card] applications, likely tens of millions of applications," according to the criminal complaint.

In a statement, Capital One said the hack affected 100 million individuals in the US and 6 million in Canada.

"Importantly, no credit card account numbers or log-in credentials were compromised and over 99 percent of social security numbers were not compromised," the bank said.

Thompson, who used the alias "erratic" in online conversations, allegedly posted several times about the theft on GitHub and on social media.

One posting on a Twitter account with the user name "erratic" read, "I've basically strapped myself with a bomb vest, f**king dropping capital ones dox and admitting it," according to the complaint.

Authorities said electronic storage devices containing a copy of the stolen data were allegedly recovered at her residence on Monday.

Capital One said some of the information in the applications stolen, such as social security numbers, is encrypted or tokenized. Other information including names, addresses, dates of birth and credit card history was not secured.

The company said it expects the breach to cost between $100 to $150 million in 2019. It added that free credit monitoring and identity protection would be made available to anyone affected. 

Thompson faces up to five years in prison and a $250,000 fine if convicted on the charge of computer fraud.