In the night between March 18th and 19th, the global aluminium producer became the target of a "massive" cyber attack involving ransom ware, forcing it to disconnect from various sites and factories and switch to manual operations in others.
The attack also forced it to postpone the publication of its quarterly earnings, originally scheduled for Tuesday, to June 5th.
"The cyber attack that hit us on March 19th has affected our entire global organization," said CEO Svein Richard Brandtzaeg in a statement.
Norsk Hydro estimated that the overall financial impact of the cyber attack would be 400-450 million Norwegian krona ($46-52 million), but also said it had "robust" cyber insurance.
The light metals producer said it had not paid any ransom and has filed a complaint with Norwegian police, which has opened an investigation.
The company's shares dropped 1.65 percent in morning trading on the Oslo Stock Exchange.