ABSTRACT. The article analyzes well-known attempts to solve the scientific problem of calculating the cybernetic stability of a special-purpose information system. It is established that at this time, existing solutions do not take into account the active actions of destructive information influences when calculating cybernetic stability, and the result is static, which reflects the state of the components of the security policy system. Of course, this is not enough to assess the real state. As a result, it became necessary to develop a diagnostic technique that would provide a calculation of the cybernetic stability of a special-purpose information system based on the results of active cybernetic actions. The mathematical apparatus of the method provides calculation of cybernetic stability of a special-purpose information system for the worst-case scenario model, for the so-called zero-day threat event.The practical significance and application lies in the practical possibility of determining the level of cybernetic stability of a special-purpose information system, taking into account the active actions of destructive information influences at the stage of design and operation of the system.Scientific novelty. The scientific novelty of the result is that a solution to the scientific and practical problem of calculating the cybernetic stability of a special-purpose information system is proposed, taking into account the active actions of destructive information influences of the zero-day threat.