Software components for on-board architectures in the space domain are increasingly reliant on Commercial Off-The-Shelf (COTS), Open Source (OSS) or other third party software products. However, these software components often have not been built with mission critical requirements in mind. Development project teams incorporating these products have limited knowledge of or control over the processes applied during the design, implementation, testing and maintenance of selected COTS/OSS software products. These constraints generate uncertainty of potential software induced failures. Moreover, the lack of information regarding security vulnerabilities increases the risks of their usage, since their exploitation might lead to undesired behaviour of the software and therefore to a system failure. The purpose of this paper is to present a combined approach that takes into account reliability and security enhancements for third party software, based on Time-Space Partitioning and Multiple Levels of Security.