Recently there has been a great need to provide an adequate security level in Cloud Environments, as they are vulnerable to various attacks. Malicious behaviors such as Denial of Service attacks, especially when targeting large-scale data management systems, cannot be detected by typical authentication mechanisms and are responsible for drastically degrading the overall performance of such systems. In this paper we propose a generic security management framework allowing providers of Cloud data management systems to define and enforce complex security policies. This security framework is designed to detect and stop a large array of attacks defined through an expressive policy description language and to be easily interfaced with various data management systems. We show that we can efficiently protect a data storage system, by evaluating our security framework on top of the BlobSeer data management platform. We evaluate the benefits of preventing a DoS attack targeted towards BlobSeer through experiments performed on the Grid'5000 testbed.