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In the last few years, the popularity of web-based applications has grown tremendously. A number of factors have led an increasing number of organizations and individuals to rely on web-based applications to provide access to a variety of services. Today, web-based applications are routinely used in security-critical environments, such as medical, financial, and military systems.

Web-based systems are a composition of infrastructure components, such as web servers and databases, and of application-specific code, such as HTML-embedded scripts and server-side CGI programs. While the infrastructure components are usually developed by experienced programmers with solid security skills, the application-specific code is often developed under strict time constraints by programmers with little security training. As a result, vulnerable web-based applications are deployed and made available to the whole Internet, creating easily exploitable entry points for the compromise of entire networks.

To ameliorate these security problems, it is necessary to develop tools and techniques to improve the security of web-based applications. The most effective approach would be to provide secure mechanisms that can be used by well-trained developers. Unfortunately, this is not always possible, and a second line of defense is represented by auditing the application code for possible security problems. This activity, often referred to as web vulnerability analysis, allows one to identify security problems in web-based applications at early stages of development and deployment.

Recently, a number of methodologies and tools have been proposed to support the assessment of the security of web-based applications. In this chapter, we survey the current approaches to web vulnerability analysis and we propose a classification along two characterizing axes: detection model and analysis technique. We also present the most common attacks against web-based applications and discuss the effectiveness of certain analysis techniques in identifying specific classes of flaws.