Hackers who stole the personal data of about 4.5 million patients of hospital group Community Health Systems Inc broke into the companys 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 Inc, David Kennedy, chief executive of TrustedSec LLC, told Reuters on Wednesday. Community Health Systems said on Monday that the attack had originated in China. Once in, they hacked their way into a database and stole millions of social security numbers and other records, he said.