Heterogeneous wireless sensor network (HWSN) based border surveillance systems are composed of several types of sensors such as video, temperature, seismic, and sound sensors. The data collected in these networks are analyzed by data aggregators or a central base station to determine illegal activities inside the border region. Due to high security requirements, concealed data aggregation is vital for mission critical HWSNs. However, since multiple sensor types that generate different data types, performing concealed data aggregation in these networks is not a trivial task. In this paper, a concealed data aggregation protocol that allows aggregation of multiple data types in border surveillance systems. The novel idea behind the paper is that the proposed concealed data aggregation protocol is able to aggregate different sensor data types into a single data packet. Moreover, the base station is able to obtain each different data type separately when it decrypts the aggregated data.