  We show that arbitrarily weak interparticle interactions destabilize the
surface states of 3D topological superconductors with spin SU(2) invariance
(symmetry class CI), in the presence of non-magnetic disorder. The conduit for
the instability is disorder-induced wavefunction multifractality. We argue that
time-reversal symmetry breaks spontaneously at the surface, so that
topologically-protected states do not exist for this class. The
interaction-stabilized surface phase is expected to exhibit ferromagnetic
order, or to reside in an insulating plateau of the spin quantum Hall effect.
