Abstract: Data platforms are at the heart of smart cities' services: they collect, process and route various types of static and dynamic data, to be used in information creation and, subsequently, as basis for the decisions of different actors. The efficient collection and transmission of real-time data is facilitated by the existence of a substantial volume of sensors installed in the infrastructure of the city. Sensors' activity is complemented by a wide range of services and applications that analyze the collected data and generate, based on them, useful information for end users: citizens, businesses, city administration and other stakeholders that co-exist in an urban environment. As a result, the smart city paradigm is associated with the technological progress made through the Internet of Things, Artificial Intelligence, Blockchain, and Big Data, leading to an informed and data-led governance. The talk overviews ethical and security-related problems in the context of smart cities, seen as huge data consumers and producers. Trends as hyper connectivity, messy complexity, loss of boundary and industrialized hacking transform smart cities in complex environments in which the already-existing security analysis are not useful anymore. Specific data-security requirements and solutions will be approached in a four-layer framework, with elements considered to be critical to the operation of a smart city: smart things, smart spaces, smart systems, and smart citizens.