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Since current Internet threats contain not only malicious codes like Trojan or worms, but also spyware and adware which do not have explicit illegal content, it is necessary to have a mechanism to prevent hidden executable files downloading in the network traffic. In this paper, we present a new solution to identify executable content for anomaly based network intrusion detection system (NIDS) based on file byte frequency distribution. First, a brief introduction to application level anomaly detection is given, as well as some typical examples of compromising user computers by recent attacks. In addition to a review of the related research on malicious code identification and file type detection in section 2, we will also discuss the drawback when applying them for NIDS. After that, the background information of our approach is presented with examples, in which the details of how we create the profile and how to perform the detection are thoroughly discussed. The experiment results are crucial in our research because they provide the essential support for the implementing. In the final experiment simulating the situation of uploading executable files to a FTP server, our approach demonstrates great performance on the accuracy and stability.