Recent developments in small satellites have led to an increasing interest in building satellite clusters as open systems that provide a "cluster-as-a-service" in space. Since applications with different security classification levels must be supported in these open systems, the system must provide strict information partitioning such that only applications with matching security classifications interact with each other. The anonymous publish/subscribe communication pattern is a powerful interaction abstraction that has enjoyed great success in previous space software architectures, such as NASA’s Core Flight Executive. However, the difficulty is that existing solutions that support anonymous publish/subscribe communication, such as the OMG Data Distribution Service (DDS), do not support information partitioning based on security classifications, which is a key requirement for some systems. This paper makes two contributions to address these limitations. First, we present a transport mechanism called Secure Transport that uses a lattice of labels to represent security classifications and enforces Multi-Level Security (MLS) policies to ensure strict information partitioning. Second, we present a novel discovery service that allows us to use an existing DDS implementation with our custom transport mechanism to realize a publish/subscribe middleware with information partitioning based on security classifications of applications. We also include an evaluation of our solution in the context of a use case scenario.