Cloud computing is a promising approach for the efficient use of computational resources. It delivers computing as a service rather than a product for a fraction of the cost. However, security concerns prevent many individuals and organizations from using clouds despite its cost effectiveness. Resolving security problems of clouds may alleviate concerns and increase cloud usage; in consequence, it may decrease overall costs spent for the computational devices and infrastructures. This paper particularly focuses on the Platform-as-a-Service (PaaS) clouds. Security of PaaS clouds is considered from multiple perspectives including access control, privacy and service continuity while protecting both the service provider and the user. Security problems of PaaS clouds are explored and classified. Countermeasures are proposed and discussed. The achieved solutions are intended to be the rationales for future PaaS designs and implementations.