Nonlinear two-photon interactions of an atomic system mediated by a thermal quantized electromagnetic field mode are investigated. We analyze the properties of the electromagnetic field inside the resonator mode. In particular, for a random particle injection rate we obtain an exact analytical solution for the steady state of the cavity field. The mean number of photons into the resonator mode is quite sensitive to the temperature. For larger atomic injection rates we find a suppression of the mean photon number into the quantized mode.