NATO's new position as of July 2016: http://www.nato.int/cps/en/natohq/topics_78170.htm

    In July 2016, Allies reaffirmed NATO’s defensive 
    mandate and recognised cyberspace as a domain of
    operations in which NATO must defend itself as 
    effectively as it does in the air, on land and at sea.

The technical verbiage used is "domain of operations" and "security domain".

This article says that "massive" cyber attacks can lead to invocation of Article 5: http://www.reuters.com/article/us-cyber-nato-idUSKCN0Z12NE

http://www.navyhistory.org/wp-content/uploads/2014/02/541-b_...