New York - Hackers who stole the personal data of about 4.5 million patients of hospital group Community Health Systems broke into the company's computer system by exploiting the "Heartbleed" internet bug, making it the first known large-scale cyber attack using the flaw, according to a security expert. The hackers, taking advantage of the pernicious vulnerability that surfaced in April, got into the system by using the Heartbleed bug in equipment made by Juniper Networks, David Kennedy, chief executive of TrustedSec, told Reuters on Wednesday. Kennedy said that multiple sources familiar with the investigation into the attack had confirmed that Heartbleed had given the hackers access to the system. Community Health Systems said on Monday that the attack had originated in China.