Today, collaboration is considered a way to improve efficiency and quality of work and this concept is spreading quickly over the technological world, giving place to the creation of ubiquitous and distributed collaborative environments where the different high level services are composed from more basic modules. A typical example of this kind of environments are the NGCWEs (Next Generation Collaborative Working Environments), which are mainly characterized by the utilization of heterogeneous and low capacity mobile devices. In this paper we take into account all the special features that characterize NGCWEs in order to propose a security model which best fits their requirements. With this aim, we have developed a solution based on symmetric cryptography, and we have introduced the concept of a centralized authorization server, in order to minimize the load that the security solution imposes over the final systems.