In the past decades, focus of computation has shifted to high performance computing like Grid Computing and Cloud Computing. In Grid computing, grid server is responsible for managing the all resources like processor, memory and CPU cycles. Grids are basically networks that pool resources, CPU cycles, storage or data from many different nodes used to solve the complex or scientific problem. However in this case, security is a major concern. Even most grid security researches focus on user authentication, authorization and secure communication. This paper presents DDoS and Data Modification attack scenario and also provides the solution to prevent it.