Security is becoming one of the major concerns for web applications and other Internet based services, which are becoming pervasive in all kinds of business models, organizations, and so on. Moreover, critical systems such as those related to health care, banking, or even emergency response, are relying on such applications and services. Web applications must therefore include, in addition to the expected value offered to their users, reliable mechanisms to ensure their security. In this paper, we focus on the specific problem of preventing crosssite scripting attacks against web applications. We present a study of this kind of attacks, and survey current approaches for their prevention. Applicability and limitations of each proposal are also discussed.