Web servers and the Web services associated with them have become increasingly important in the last few years. Online banking, e-mail, and money, business- to-business (B2B), and business-to-client (B2C) transactions are growing rapidly. It is difficult to imagine modern business without these forms of networking. However, there are also significant negative aspects. In many cases, due to competitive pressures, companies and government agencies had to implement these services very fast, often too fast and without any appreciation of the concepts of security and protection. As a consequence, it turns out that a hacker can misuse with little effort these Web services or compromise the underlying database (e.g., to obtain access to credit cards numbers or social insurance information). A very significant percentage of the population in developed and developing countries is using wired and wireless connections for reading e-mails, accessing newsgroups, or using Internet banking. All these services are running on a Web server. Most Web servers are running the Apache or the Microsoft Internet Information Server (IIS) (all versions of both servers [Apache 1.3.x/2.x, IIS 3-6]) (Netcraft, 2006). Of these, older versions of the Internet Information Server are especially vulnerable to numerous attacks. Therefore, an attacker is in a position to break, with little effort, into many Web servers running IIS 4 or 5. However, the Apache Web server (running on Windows systems) is also vulnerable to similar attacks. Moreover, using a Web server based on UNIX or Linux is not a guarantee for a secure system. UNIX and Linux systems are also affected by inherent weaknesses and vulnerabilities such as buffer overflows and the handling of format strings (ZDNet, 2006). Readers who like to have more general insight are referred to works by Leiss (1990) and Garfinkel and Spafford (2002). These books give broader perspectives on Internet security.