An implemented system for on-line analysis of multiple distributed data streams is presented. The system is conceptually universal since it does not rely on any particular platform feature and uses format adaptors to translate data streams into its own standard format. The system is as powerful as possible (from a theoretical standpoint) but still efficient enough for on-line analysis thanks to its novel rule-based language (BUS-SEL) which is specifically designed for efficientpro-cessing of sequential unstructured data streams. In this paper, the generic concepts are applied to security audit trail analysis. The resulting system provides powerful network security monitoring and sophisticated tools for intrusion/anomaly detection. The rule-based and command languages are described as well as the distributed architecture and the implementation. Performance measurements are reported, showing the effectiveness of the approach.