Experience of recent years indicates that cyberattacks are often aimed at informational systems of state bodies, healthcare, energy, financial and transport sector, etc. with increasing, unpredictable consequences. The 2012 National Security Strategy of Ukraine includes a provision on establishing a national cybersecurity system. Its effective implementation requires to take into account not only threats of a criminal nature, but a full range of threats with varying origin, tools used, targets and purpose. This paper presents an analysis of the current state of threats, related to transnational cybercrime and attempts to use modern informational technologies by foreign governments, organisations and individuals. The national cybersecurity system—the set of administrative, legal, technical measures related to informational security and data protection—continues to be one of the key elements to guarantee national security. This article is focused on the cyber dimension of the national security, in particular its legal aspect in context of an ongoing defence and security sector reform.