Telehealth systems (TS's) provide remote health-based services to improve the quality of service of patient treatment. Most healthcare professionals have access to standard telecommunications technology (such as Wireless Body Area Network (WBAN), biosensors, remote medical robots, and others) to offer remote care of elderly and physically less able patients as well as remote surgeries, treatments, and diagnoses. In order to ensure the functionality of TS's, several systemic properties must be satisfied, including security. Although there are studies that discuss different security approaches in TS's, it is difficult to have a clear view of existing security issues and solutions for these systems. In this article, a systematic mapping study was performed to detect, organize and characterize security issues in TS's. We identified 41 studies which were classified according to their research strategy, target problem, security issue addressed, and proposals. Results reveal that (i) 4 security issues were identified; (ii) 3 strategies were distinguished to handle security issues; (iii) patient and wireless medical data are the most affected medical supplies. Security in TS's reveals diverse challenges that concern Software Engineering. Areas such as requirements, software architecture, and security patterns play an important role in order to handle security issues.