  Reissner--Nordstr\"om black holes have two static regions:
  $r > \ro$ and $0 < r < \ri$, where $\ri$ and $\ro$ are the inner and outer
horizon radii. The stability of the exterior static region has been established
long time ago. In this work we prove that the interior static region is
unstable under linear gravitational perturbations, by showing that field
perturbations compactly supported within this region will generically excite a
mode that grows exponentially in time. This result gives an alternative reason
to mass inflation to consider the space time extension beyond the Cauchy
horizon as physically irrelevant, and thus provides support to the strong
cosmic censorship conjecture, which is also backed by recent evidence of a
linear gravitational instability in the interior region of Kerr black holes
found by the authors. The use of intertwiners to solve for the evolution of
initial data plays a key role, and adapts without change to the case of
super-extremal \rn black holes, allowing to complete the proof of the linear
instability of this naked singularity. A particular intertwiner is found such
that the intertwined Zerilli field has a geometrical meaning -it is the first
order variation of a particular Riemann tensor invariant-. Using this,
calculations can be carried out explicitely for every harmonic number.
