Reliable storage of large amounts of data is always a delicate issue. Availability, efficiency, data integrity, and confidentiality are some features a data backup system should provide. At the same time, corporate computers offer spare disk space and unused networking resources. In this paper, we propose an intrusion-tolerant cooperative backup system that provides a reliable collaborative backup resource by leveraging these independent, distributed resources. This system makes efficient use of network and storage resources through compression, encryption, and efficient verification processes. It also implements a protocol to tolerate Byzantine behaviors, when nodes arbitrarily deviate from their specifications. Experiments performed to evaluate the proposal showed its viability.