The success of the CubeSat nanosatellites space missions depends on all systems ability to perform properly in a harsh environment. A key component in every space mission is the flight software, which manages all the processes that must be performed by the satellite on its onboard computer. Literature shows that CubeSat missions suffer high infant mortality and many spacecraft failures are related to flight software errors, some of them resulting in a complete mission loss. Extensive software testing is the primary tool used by flight software developers, to ensure code quality and avoid such failures. Nevertheless, CubeSat developers tend to use COTS or flight-proven solutions which usually have low testing coverage. Nowadays, there is still some pending matter in the field of testing nanosatellites flight software and some of the most used solutions do not even report unit tests. To overcome the agile CubeSat development versus delivering quality software trade-off, we propose the use of fuzz testing techniques applied to the SUCHAI series of nanosatellites, being developed at the University of Chile. The successful application of this technique allowed us to find and solve many bugs not covered by classic strategies, such as unit testing and software in the loop simulation.