  A search for pair production of the supersymmetric partner of the top quark,
the top squark, in proton-proton collision events at $\sqrt{s} =$ 13 TeV is
presented in a final state containing hadronically decaying tau leptons and
large missing transverse momentum. This final state is highly sensitive to
high-$\tan{\beta}$ or higgsino-like scenarios in which decays of electroweak
gauginos to tau leptons are dominant. The search uses a data set corresponding
to an integrated luminosity of 77.2 fb$^{-1}$, which was recorded with the CMS
detector during 2016 and 2017. No significant excess is observed with respect
to the background prediction. Exclusion limits at 95% confidence level are
presented in the top squark and lightest neutralino mass plane within the
framework of simplified models, in which top squark masses up to 1100 GeV are
excluded for a nearly massless neutralino.
