Malicious code still represents the major threat for the information security and this tendency is not likely to disappear in the near future. One of the reasons responds to the fact that security companies have been trying to overcome this problem through the development of detective security tools, which base their success on the detection of well-known attacks. The new era of malware has made detective tools not useful to offer an acceptable level of security, exposing the information to critical threats. This book presents a realistic scenario where one of the most common organizational security architectures is tested against sophisticated malware attacks. The intention of this exercise is to identify important weaknesses that today organizational countermeasures present when trying to prevent malicious code. Additionally, an analysis of today proactive tools against the attacks used in the scenario previously described is shown. At the end, the reader will have a clear understanding of the threat posed by today's malware attacks and will be aware of host based proactive tools as one of the most effective countermeasures to face this problem.