A significant number of risk factors affect the automated spacecraft flight control system (ASFCS). To effectively neutralize these factors, it is necessary to assess the sensitivity and stability of the information security facility of the ASFCS. For different security classes of such systems, it is necessary to define basic functional safety indicators. We rely on the notion of security functions durability, and for its evaluation we introduce strict definitions of the basic concepts: the mechanism of protection, security facility, reliability of control, sensitivity and durability of information security facility. For the security coefficient, which is an indicator of the durability of information security facility, we obtained an analytical expression. Using the standard model of the threat counteraction process, we solved the task of determining some tentative values of type 2 error probabilities for the security facility. Furthermore, we assessed the priorities of the information security facility, which enabled us to obtain a variational series of type 2 error probability values, and in certain cases to set the required values of such probabilities of the security facility. The application of the developed method makes it possible to assess the residual threat impact on the information resources of the automated spacecraft flight control system. If the residual risk is acceptable, then the stability of the protection mechanisms meets the requirements of the system's safety. Otherwise, it is necessary to use protection mechanisms with the increased durability.