  Light bosons can mediate long-range forces. We show that light bosonic
mediators interacting with a background medium, in particular, with the cosmic
neutrino background (C$\nu$B), may induce medium-dependent masses which could
effectively screen long-range forces from detection. This leads to profound
implications for long-range force searches in e.g. the E\"ot-Wash, MICROSCOPE,
and lunar laser-ranging (LLR) experiments. For instance, we find that when the
coupling of the mediator to neutrinos is above $3\times10^{-10}$ or
$5\times10^{-13}$, bounds from LLR and experiments employing the Sun as an
attractor, respectively, would be entirely eliminated. Larger values of the
coupling can also substantially alleviate bounds from searches conducted at
shorter distances.
