Due to their recent appearance and the reduced requirements in terms of network bandwidth, Slow Denial of Service Attacks detection represents a particularly challenging problem. This paper presents a novel detection method, analyzing spectral features of the network traffic over small time horizons. The proposed method has been validated by extrpolating data referred to real traffic traces, elaborated over the Local Area Network of our research institute. We have considered different kinds of attacks and results show how the proposed approach is reliable and applicable also in other cybersecurity contexts.