The linear theory of the dynamic instability of strong plane-parallel or spherical adiabatic blast shock wave in a medium with initially uniform density is considered. The principal frequency eigenvalues are obtained as a function of transverse wavelength for several values of the adiabatic index. It is found that the plane-parallel and spherical shocks are overstable for the adiabatic index less than about 1.2. The astrophysical implications of these results are discussed. These detailed eigenvalue and eigenmode results provide an opprotunity to test the accuracy of three-dimensional numerical models of shock wave evolution.open7