  The CRESST (Cryogenic Rare Event Search with Superconducting Thermometers)
dark matter search experiment aims for the detection of dark matter particles
via elastic scattering off nuclei in $\mathrm{CaWO_4}$ crystals. To understand
the CRESST electromagnetic background due to the bulk contamination in the
employed materials, a model based on Monte Carlo simulations was developed
using the Geant4 simulation toolkit. The results of the simulation are applied
to the TUM40 detector module of CRESST-II phase 2. We are able to explain up to
$(68 \pm 16)\,\mathrm{\%}$ of the electromagnetic background in the energy
range between $1\,\mathrm{keV}$ and $40\,\mathrm{keV}$.
