In the second half of the 20th century, humanity went through an unprecedented technical and technological development. As a result, technological innovations emerged in the course of the last third of the century which have now become indispensable parts of everyday life, the whole society and even the state. Among them, we must mention the IT sector, which has effectively enabled global contacts and communication between people and organizations across different parts of the world through various tools, programs and networks. Moreover, it also facilitates and simplifies everyday tasks both in the private and the public sector. Cyberspace is a unique and complex phenomenon, since it can be described with physical and geographical concepts, but in addition, its virtual features also have extraordinary relevance. As a result of its remarkable expansion, fundamental areas such as sociology, geopolitics, security policy or warfare must also be reconsidered. This paper provides an overview of the new types of security challenges for the 21st century, most notably security risks related to the cyberspace. In addition, some aspects of cyber warfare, such as cyber intelligence, cyber attack and cyber defense are examined. Particular attention is given to the question whether a cyber attack in itself can reach the level of an armed attack, and if so, what means can be used by the State under attack in defense.