  It has long been suggested that solutions to linear scalar wave equation
$$\Box_g\phi=0$$ on a fixed subextremal Reissner-Nordstr\"om spacetime with
non-vanishing charge are generically singular at the Cauchy horizon. We prove
that generic smooth and compactly supported initial data on a Cauchy
hypersurface indeed give rise to solutions with infinite nondegenerate energy
near the Cauchy horizon in the interior of the black hole. In particular, the
solution generically does not belong to $W^{1,2}_{loc}$. This instability is
related to the celebrated blue shift effect in the interior of the black hole.
The problem is motivated by the strong cosmic censorship conjecture and it is
expected that for the full nonlinear Einstein-Maxwell system, this instability
leads to a singular Cauchy horizon for generic small perturbations of
Reissner-Nordstr\"om spacetime. Moreover, in addition to the instability
result, we also show as a consequence of the proof that Price's law decay is
generically sharp along the event horizon.
