The cloud heralds a new era of computing where application services are provided through the Internet. Cloud Computing is a flexible, cost-effective, and proven delivery platform for providing business or consumer IT services over the Internet. The computing capability of mobile systems is enhanced by Cloud computing. Mobile devices can rely on cloud computing and information storage resource, to perform computationally intensive operations such as searching, data mining, and multimedia processing. Along with traditional computation services it provides, mobile cloud also enhances the operation of traditional ad hoc network by treating mobile devices as service nodes, e.g., sensing services. The sensed information, such as location coordinates, health related information, should be processed and stored in a secure fashion to protect user's privacy in the cloud. While the economic ease for cloud computing is compelling, the security challenges it poses are equally striking. The security threats have become obstacles in the rapid adaptability of the mobile cloud computing paradigm. Significant efforts have been devoted in research organizations and academia to build secure mobile cloud computing environments and infrastructures. In spite of the efforts, there are a number of loopholes and challenges that still exist in the security policies of mobile cloud computing. We discuss these issues here, identifying the main vulnerabilities in this kind of systems and the most important threats found in the literature related to Cloud Computing and its environment as well as to identify and relate vulnerabilities and threats with possible solutions.