In this paper we present a steganalytic attack against mp3stego, a publicly available open-source steganographic tool which uses mp3 audio files as the stego medium to embed hidden information. We first identify through extensive statistical experimentation which parameters of the medium suffer a significant deviation from normality, then we construct a very efficient blind steganalytic algorithm based on their use. We finally present the results obtained with this steganalytic approach over various sets of mp3 files, and an algorithm for the estimation of the amount of hidden data.