Cloud computing is a technology helps the user to keep up data and its application by using internet and central remote servers. Cloud computing has greater flexibility and availability at lower cost. Software-as-a-Service (SaaS) cloud systems allow application service providers to deliver their applications via massive cloud computing infrastructures. SaaS is becoming an increasingly prevalent delivery model as underlying technologies that support web services and Service-Oriented Architecture (SOA) mature and new developmental approaches, such as Ajax, become popular. SaaS clouds are vulnerable to malicious attacks because of their sharing nature.