Unattended Wireless Sensor Networks (UWSNs), characterized by the absence of real-time communication between sensors and sinks, impose sensors to retain data till the next visit of a mobile sink to off-load their data. In such networks, if a sensor is compromised, data accumulated in the sensor are exposed to attackers. In addition, by holding the secret key of the compromised sensor, attackers can also learn post-compromise data accumulated by the sensor. Furthermore, once sensors stop working due to, for instance, node crash or battery depletion, all the accumulated data will be lost. In this paper, we propose a secure and reliable data distribution scheme that addresses these challenges. Detailed analysis shows that our scheme can provide forward secrecy, probabilistic backward secrecy and data reliability. To further improve probabilistic backward secrecy and data reliability, a constrained optimization data distribution scheme is also proposed. Detailed analysis and simulation results show the superiority of the proposed scheme in comparison with several existing schemes developed for UWSNs.