The initial attack occurred in December 2013, but the company did not learn about it until Aug. 5. Since then it has been working with the FBI and cybersecurity firm Mandiant to investigate the breach.
The hackers may have had access to customer records which include names, addresses, telephone numbers, dates of birth, Social Security numbers, member identification numbers, financial accounts and medical claims information.
Records may contain all or just some of that information, depending on the customer's relationship with the company. The breach doesn't affect just Excellus members, but also members of other Blue Cross Blue Shield plans who sought medical treatment in the upstate New York area serviced by the company.
The information was encrypted, but the attackers gained administrative privileges to the IT systems, allowing them to potentially access it, the company said on a website that was set up to provide information about the incident.
No evidence has been found yet that the data was copied or misused by the attackers.
Excellus will send breach notification letters via mail to all affected persons throughout the month and is offering free credit monitoring and identity protection services for two years through a partner.
The company will not contact affected individuals via email or telephone, so any emails or phone calls claiming to be from the company in regard to this attack should be ignored as they are probably scams.
After a major data breach, it is common for cybercriminals to send emails that seek personal and financial information or which carry malicious attachments.
The incident comes after three other Blue Cross Blue Shield health insurers -- Anthem, Premera and CareFirst -- announced large data breaches this year as a result of cyberattacks.
Security researchers from Symantec attributed the Anthem breach to a sophisticated cyberespionage group of Chinese origin dubbed Black Vine that has access to exploits for previously unknown vulnerabilities. Bloomberg reported that the same group was responsible for the breach at the U.S. Office of Personnel Management (OPM) that affected 21.5 million current and former U.S. government employees.
Excellus said that it doesn't have sufficient information about the Anthem, Premera and CareFirst investigations in order to comment about possible connections between those attacks and the one against its own systems.