In this paper we propose a new model-based framework for testing security properties of Internet of Things in Smart Cities. In general a model-based approach consists in extracting test cases from a formal specification either of the system under test or the environment of the considered system in an automatic fashion. Our framework is mainly built on the use of two formalisms namely Attack Trees and Price Timed Automata. An attack tree allows to describe the strategy adopted by the malicious party which intends to violate the security of the considered IOT system. An attack tree is translated into a network of price timed automata. The product of the constructed price timed automata is then computed using the well known UPPAALL platform. The obtained timed automata product serves as input for the adopted test generation algorithm. Moreover our framework takes advantage of the use of the standardized specification and execution testing language TTCN-3. With this respect, the obtained abstract tests are translated into the TTCN-3 format. Finally we propose a cloud-oriented architecture in order to ensure test execution and to collect the generated verdicts.