The hack was announced earlier this week, although no medical records were affected

America’s second largest chain of for-profit hospitals has reported a data breach of information belonging to some 4.5 million patients, with security experts claiming that the Heartbleed bug is to blame.

Community Health Systems (CHS) announced that they had been hacked at the beginning of the week, with the stolen information including patient names, addresses, birth dates, social security numbers and phone numbers – but no medical records. CHS say that Chinese hackers were responsible.

The Heartbleed bug was first uncovered in April this year after a joint research project between Google and Finnish security group Codenomicon. The bug was a flaw in the commonly-used encryption standard OpenSSL – familiar to most internet users as the extra layer of security represented by the padlock icon in their browser.

Read more: What is Heartbleed? Am I at risk?

Webmasters and software developers were encouraged to fix the flaw as soon as possible, but many companies were criticized for taking weeks to do so.

In the case of Community Health Systems, it’s thought that the breach was due to just such a delay, with TrustSec chief executive David Kennedy telling Bloomberg that the hackers had taking advantage of network products made by a firm named Juniper.

The use of Heartbleed has not been confirmed by CHS but if true it would be the largest hacks using the flaw to date. Previous attacks taking advantage of the bug have targeted the Canadian tax authority and UK parenting site Mumsnet.