  The HADES experiment at GSI has recently provided data on the flow
coefficients $v_1,...,v_4$ for protons in Au+Au reactions at $E_{\rm lab} =
1.23$~$A$GeV (or $\sqrt{s_\mathrm{NN}}=2.4$ GeV). This data allows to estimate
the shear viscosity over entropy ratio, $\eta/s$ at low energies via a coarse
graining analysis of the UrQMD transport simulations of the flow harmonics in
comparison to the experimental data. By this we can provide for the first time
an estimate of $\eta/s\approx0.65\pm0.15$ (or $(8\pm2)\,(4\pi)^{-1}$) at such
low energies.
