Internet security problems are still a big challenge as there are many security events occurred, such as Internet worms, Spam and phishing attacks etc. Botnet, a well-organized distributed network attack, consists of a large volume of bots, which generates huge volumes of spam or launching Distributed Denial-of-Service (DDoS) attacks to victim hosts. To address these problems, a practical Collaborative Network Security Management System is proposed with well-deployed collaborative UTM (Unified Threat Management) and traffic probers. In this paper, we propose a design and implementation of cloud based Security Center for network security forensic scrutiny. We propose to use cloud storage to keep collected traffic data and processing it with cloud computing platform to find the malicious attacks. A workable case, phishing attack forensic analysis is presented and the required computing and storage resources are evaluated based on real trace data.