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In recent years, an increasing number of security threats have brought a serious risk to the internet and computer networks. Intrusion Detection System (IDS) plays a vital role in detecting various kinds of attacks. Developing adaptive and flexible oriented IDSs remains a challenging and demanding task due to the incessantly appearance of new types of attacks and sabotaging approaches. In this paper, we propose a novel unsupervised statistical approach for detecting network based attacks. In our approach, patterns of normal and intrusive activities are learned through finite generalized Dirichlet mixture models, in the context of Bayesian variational inference. Under the proposed variational framework, the parameters, the complexity of the mixture model, and the features saliency can be estimated simultaneously, in a closed-form. We evaluate the proposed approach using the popular KDD CUP 1999 data set. Experimental results show that this approach is able to detect many different types of intrusions accurately with a low false positive rate.