(Reuters) – Target Corp plans to make significant changes in light of the data breach over the holiday shopping season when hackers stole personal information of millions of customers, the U.S. retailers chief executive said in an interview to CNBC. Malware was installed on the companys point of sale registers, Gregg Steinhafel said, and Target was working with law enforcement agencies to determine who did it and when it was done. Target said on Friday an investigation found that hackers stole the personal information of at least 70 million customers, including names, mailing addresses, telephone numbers and email addresses. Steinhafel told CNBC on Sunday that Targets environment was secure.