A new approach to the description of the resonance properties of objects (vehicle compartments, cases of electronic equipment, connecting cables, etc.) in the problems of the analysis of electromagnetic compatibility and electromagnetic protection is proposed. The essence of the approach is to create worst-case adaptive models that adjust (adapt) a priori unknown resonance frequencies of the simulated objects for each input stimulus in such a way as to provide the worst value of the criterion of electromagnetic compatibility (or protection criterion). Within the framework of the proposed approach, a worst-case adaptive model of the penetration of electric and magnetic fields into a shielding enclosure (e.g., into a vehicle compartment) is developed; the model is based on the existing non-adaptive model using analytical expressions for the field inside a rectangular waveguide. The developed model is validated by the following example: the impact of external electromagnetic pulses (the radar pulse and E1 HEMP) on a vehicle (jeep) is analyzed; the results of the calculation by the developed model are compared with the results of the FDTD calculation.