  Weak gravitational lensing is one of the key probes of cosmology. Cosmic
shear surveys aimed at measuring the distribution of matter in the universe are
currently being carried out (Pan-STARRS) or planned for the coming decade (DES,
LSST, EUCLID, WFIRST). Crucial to the success of these surveys is the control
of systematics. In this work a new method to constrain one such family of
systematics, known as multiplicative bias, is proposed. This method exploits
the cross-correlation between weak lensing measurements from galaxy surveys and
the ones obtained from high resolution CMB experiments. This cross-correlation
is shown to have the power to break the degeneracy between the normalization of
the matter power spectrum and the multiplicative bias of cosmic shear and to be
able to constrain the latter to a few percent.
