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Web applications typically employ sanitization functions to sanitize user inputs, independently whether this input is assumed to be legitimate, invalid or malicious. When such functions do not work correctly, a web application immediately becomes vulnerable to security attacks such as XSS. In this paper, we report a combinatorial approach to analyze XSS vulnerabilities in web applications. Our approach first performs combinatorial testing where a set of test vectors is executed against a subject application. If one or more XSS vulnerabilities are triggered during testing, we analyze the structure of each test vector to identify XSS-inducing combinations of its parameter model. If an attack vector contains an XSS-inducing combination, then the execution of this vector will successfully exploit an XSS vulnerability. Identification of XSS-inducing combinations provides insights about which kinds of user input might still be leverageable for XSS attacks and how to correct the function to provide better security guarantees. We conducted an experiment in which our approach was applied to four sanitization functions from the Web Application Vulnerability Scanner Evaluation Project (WAVSEP). The experimental results show that our approach can effectively identify XSS-inducing combinations for these sanitization functions.

The input space of a system must be modeled before combinatorial testing can be applied to this system. The effectiveness of combinatorial testing to a large extent depends on the quality of the input space model. In this paper we introduce an input space modeling methodology for combinatorial testing. The main idea is to consider the process of input space modeling as two steps, namely, input structure modeling and input parameter modeling. The first step tries to capture the structural relationship among different components in the input space. The second step tries to identify parameters, values, relations and constraints for individual components. We also suggest strategies about how to perform unit and integration testing based on the input space structure. We applied the proposed methodology to two real-life programs. We report our experience and results that demonstrate the effectiveness of the proposed methodology.

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