Dark matter particles with masses up to 100 MeV and couplings to ordinary matter are strongly constrained by observations of SN1987A. While these limits are commonly studied for light bosons, in this talk I will discuss the constraints on dark fermions interacting with electrons, muons or neutrinos through a massive vector mediator as derived in arXiv:2307.03143. After a pedagocial introduction I will set up the basic formalism and present limits in the general parameter space for different regimes of the mediator mass, also using simple analytic approximations.