In the face of what has being called Norway's "largest coordinated hacker attack," said the Norwegian government has launched its largest-ever campaign to warn the country's oil and energy companies of potential cyber attacks.

As many as 50 companies, including the country's main electricity grid owner, were already targeted and more attacks could follow. International sources told Norway's national security authority of the attacks and potential system breaches.

The attackers are using spear phishing tactics to send emails with malicious attachments to important company employees. Once an employee opens an attachment, a program scans the user's system for possible security lapses. Upon finding a vulnerability, the program begins communicating with the attackers who can then work remotely to build up an attack.

The government warning offers specific instructions about where to look in company logs for evidence of attacks.