U.S. hospital operator Community Health Systems (CHS) announced today that it became a victim of a hacking attack involving personal information on 4.5 million of its patients.

CHS, an operator of 206 medical facilities in 29 states across the U.S., filed a document with the U.S. Securities and Exchange Commission (SEC) saying a data breach hit the firm in April and June this year. According to a CNBC report, the group of hackers is believed to be based in China and is classified under "Advanced Persistent Threat."

Like Us on Facebook

In the regulatory filing with the SEC, the hospital operator said the attackers breached its data security system with the use of highly sophisticated technology and malware, an AFP report detailed.

Upon its cooperation with computer security specialists, CHS also came to the conclusion that the hackers were from China and their main goal was to steal information rather than destroy computer systems.

CHS also said the data stolen included patients' names, birthdates, addresses, contact numbers and Social Security numbers. However, the hackers were not able to get any of the patients' medical and credit card information.

In the past five years, these patients sought medical services from doctors affiliated with the firm, CHS added.

The medical firm offered to provide the affected patients protection against identity theft.

Security group Mandiant was the first one to use the term "advanced persistent threat" last year when Chinese military personnel were accused of targeting the computer networks of companies based in Canada, the U.S. and the U.K.

The Chinese group's hacking activities were traced all the way back to 2006 and its operations were detailed in a major report.

Upon detecting the data breach, CHS said it hired Mandiant to look into the incident.

It is not yet confirmed if Mandiant thinks the same Chinese military group is responsible for the CHS hacking or not. The company has yet to comment on the issue.

Mandiant is a unit of security firm FireEye.

TagsCommunity Health Systems, U.S., Securities and Exchange Commission, AFP, CNBC, Mandiant, FireEye, U.K.