Cyber-Physical Systems (CPS) are driving the fourth industrial revolution. In CPS, uncertainty is inherent that may cause security issue either directly or indirectly. To understand security together with uncertainty, we propose SUnCPS, a taxonomy of security-related uncertainty in the context of CPS. SUnCPS can provide a structured representation of security-related uncertainty in CPS as the basis for different (early) security engineering activities for CPS such as security risk analysis and management, vulnerability/attack analysis, and security testing for CPS.