International audienceThis paper introduces an attack detection and response system based on multi-level rule expression language. It provides a framework to evaluate, identify, classify and defend against sophisticated attacks. Our approach helps simplifying complex rules' expression and alert handling, thanks to a modular architecture and an intuitive rules along with a powerful expression language. The proposed system is flexible and takes into account several attack properties in order to simplify attack handling and aggregate defense mechanisms. 1 Introduction Security aims at protecting firm resources from undesired access by users and applications. Improving security in enterprise information system relies on analyzing threats, risks and vulnerabilities to specify appropriate countermeasures. This imposes several challenges to tackle with security issues. One of these challenges is detection and mitigation of attacks. To deal with the growing complexity of new attacks, several solutions such as intrusion detection and prevention systems (IDS/IPS) and web application firewalls (WAF) have been proposed. These solutions can be based either on signature or on behavior detection. They play an important role in countering security threats. Signature based system tend to use static rules and to detect only specific attacks or anomalous behaviors that are already known. In anomaly-based case, they need learning process and detection is more complex. In addition, attack detection techniques are far from being satisfactory [1]. In fact, solutions like IDSs provide unmanageable amount of " false positives " alarms which are hard to inspect. Furthermore, many detection systems do not offer an appropriate compromise between acceptable performance and detection language simplicity