Microservice architectures (MSA) are becoming trending alternatives to existing software development paradigms notably for developing complex and distributed applications. Microservices emerged as an architectural design pattern aiming to address the scalability and ease the maintenance of online services. However, security breaches have increased threatening availability, integrity and confidentiality of microservice-based systems. A growing body of literature is found addressing security threats and security mechanisms to individual microservices and microservice architectures. The aim of this study is to provide a helpful guide to developers about already recognized threats on microservices and how they can be detected, mitigated or prevented; we also aim to identify potential research gaps on securing MSA. In this paper, we conduct a systematic mapping in order to categorize threats on MSA with their security proposals. Therefore, we extracted threats and details of proposed solutions reported in selected studies. Obtained results are used to design a lightweight ontology for security patterns of MSA. The ontology can be queried to identify source of threats, security mechanisms used to prevent each threat, applicability layer and validation techniques used for each mechanism. The systematic search yielded 1067 studies of which 46 are selected as primary studies. The results of the mapping revealed an unbalanced research focus in favor of external attacks; auditing and enforcing access control are the most investigated techniques compared with prevention and mitigation. Additionally, we found that most proposed solutions are soft-infrastructure applicable layer compared with other layers such as communication and deployment. We also found that performance analysis and case studies are the most used validation techniques of security proposals.