In recent years, cyber-physical systems (CPS) have been to many vital areas, including medical devices, smart cars, industrial systems, energy grid, etc. As these systems increasingly rely on Internet, ensuring their security requirements, which are different from those of ordinary information technology systems, has become an important research topic. However, the area is not yet well structured and, therefore, it is essential to review and analyze recent work in this field to better understand the adopted approaches and also to identify corresponding gaps and future interesting research directions. This article is based on a systematic literature review that addresses core issues in CPS security. Part of the focus is placed on the defense mechanisms introduced to help the system fully recover from attacks. However, the framework devoted to risk/threat assessments has also been highlighted, as there is a substantial need to strengthen the systems architecture to be more proactive. In addition, the ethical and societal impact of CPS is emphasized since it is essential to get a vision of the unintended outcomes of the possible evolution of these systems.