A lean premixed swirl burner configuration has been investigated numerically regarding its vulnerability with respect to flame flashback caused by combustion induced vortex breakdown (CIVB). The flashback phenomenon has been simulated using a simplified quasi two-dimensional URANS method to cover a large range of operational conditions and using Large-Eddy-Simulation to achieve a detailed view of the processes involved. The Lindsted-Vaos turbulent premixed combustion model supplemented with quenching models has been used in URANS simulations. In the LES the combustion has been modeled with a thickened flame model using a one-step reaction mechanism. The isothermal flow and the stable flame were validated and flashback limits for different operational conditions were determined.