In Fifth-Generation (5G) and Beyond 5G (B5G) networks, Network Slicing (NS) enables diverse services to coexist on shared infrastructure, but assessing Security Posture (SP) presents significant challenges due to complex, multi-dimensional security attributes and inherent uncertainty in evaluation processes. Traditional security assessment methods rely on binary approaches that fail to capture complex relationships between security dimensions. To address these limitations, this paper proposes a Fuzzy-based System for Assessment of Security Posture (FSASP) in 5G/B5G NS environment. The FSASP integrates three parameters: Identity and Access Management Strength (IAMS), Vulnerability Exposure (VE), and Monitoring and Response Maturity (MRM) and utilize Fuzzy Logic (FL) to provide comprehensive SP assessment. Simulation results demonstrate that VE has the most negative impact on SP, while achieving good security requires optimization across all dimensions. Single-dimension strengths cannot compensate the weaknesses of other parameters, emphasizing the importance of balanced security investments. The proposed FSASP offers an interpretable and uncertainty-aware framework that provides valuable guidance for security administrators in prioritizing investments and making risk mitigation decisions in dynamic 5G/B5G NS environments.