  A variety of beyond the standard model scenarios contain very light hidden
sector U(1) gauge bosons undergoing kinetic mixing with the photon. The
resulting oscillation between ordinary and hidden photons leads to spectral
distortions of the cosmic microwave background. We update the bounds on the
mixing parameter $\chi_0$ and the mass of the hidden photon $m_{\gamma'}$ for
future experiments measuring CMB spectral distortions, such as PIXIE and
PRISM/COrE. For $10^{-14}\;{\rm eV}\lesssim m_{\gamma'}\lesssim 10^{-13}\;{\rm
eV}$, we find the kinetic mixing angle $\chi_0$ has to be less than $10^{-8}$
at 95\% CL. These bounds are more than an order of magnitude stronger than
those derived from the COBE/FIRAS data.
