1. A method for a speaker verification system to prevent malicious attacks on protected resources, comprising: extracting a plurality of speech features from multiple segments of a speech signal, the speech signal being a synthetic speech signal generated by transforming voice or text with voice characteristics of a user, the speech signal enabling access to protected resources by a requestor; analyzing the plurality of speech features to determine whether the plurality of speech features exhibit periodic variation behavior, the analyzing comparing the plurality of speech features at edges of speech frames with the plurality of speech features in middles of the speech frames, the analyzing including: generating a representation of variability of the plurality of speech features; and performing a periodicity analysis of the generated representation of variability to determine that the generated representation of variability exhibits periodic variation behavior; and denying the requestor access to the protected resources based on a determination that the speech signal is a synthetic speech signal based on the exhibited periodic variation behavior.