Target CEO Gregg Steinhafel has announced plans to step down, about five months after the retailer revealed a massive breach that affected up to 70 million customers.

Steinhafel will step down from his positions as chairman of the Target board of directors, president, and CEO. CFO John Mulligan will serve as interim president and CEO until a replacement is found.

In a Monday letter to the board, Steinhafel said that the data breach "tested Target in unprecedented ways."

"From the beginning, I have been committed to ensuring Target emerges from the data breach a better company, more focused than ever on delivering for our guests," he wrote. "We have already begun taking a number of steps to further enhance data security, putting the right people, processes, and systems in place."

With those milestones in place, "now is the right time for new leadership at Target," Steinhafel said.

Target first disclosed the breach in December, noting it affected those who used credit or debit cards in its U.S. stores between Nov. 27 to Dec. 15. In addition to nabbing 40 million credit and debit card numbers, the hackers managed to steal the personal information of up to 70 million individuals, Target said. That includes customers' names, mailing addresses, phone numbers, and email addresses.

In March, a Bloomberg BusinessWeek article said that Target ignored warnings about a possible intrusion. The report says Target used a malware detection tool from FireEye, and that the product picked up on sketchy behavior in late November. Target was notified "and then ... nothing happened," according to BusinessWeek.

At the time, Target said that its initial investigation "determined that [the FireEye warnings] did not warrant immediate follow up." But "with the benefit of hindsight, we are investigating whether, if different judgments had been made the outcome may have been different."