We present a classifier ensemble system using a combination of Neural Networks and rule-based systems as base classifiers that is capable of detecting network-initiated intrusion attacks on web servers. The system can recognize novel attacks (ie, attacks it has never seen before) and categorize them as such. The performance of the Neural Network in detecting attacks from network data alone is very good with success rates of more than 78% in recognizing new attacks but suffers from high false alarms rates. An ensemble combining the original ANN with a second component that monitors the server’s system calls for detecting unusual activity results in high prediction accuracy with very small false alarm rates. We experiment with a variety of ensemble classifiers and decision making schemes for final classification. We report on the results we got from our approach and future directions for this research