Functionality is the highest semantic level of the software behavior pyramid that reflects goals of the software rather than its specific implementation. Detection of malicious functionalities presents an effective way to detect malware in behavior-based IDS. A technology for mining system call data, discussed herein, results in the detection of functionalities representing operation of legitimate software within a closed network environment. The set of such functionalities combined with the frequencies of their execution constitutes a normalcy profile typical for this environment. Detection of deviations from this normalcy profile, new functionalities and/or changes in the execution frequencies, provides evidence of abnormal activity in the network caused by malware. This approach could be especially valuable for the detection of targeted zero-day attacks. The paper presents the results of the implementation and testing of the described technology on the computer network testbed.