  It has been argued that the flux anomalies detected in gravitationally lensed
QSOs are evidence for substructures in the foreground lensing haloes. In this
paper we investigate this issue in greater detail focusing on the Cusp relation
which corresponds to images of a source located to the cusp of the inner
caustic curve. We use numerical simulations combined with a Monte Carlo
approach to study the effects of the expected power law distribution of
substructures within LCDM haloes on the multiple images.
  Generally, the high number of anomalous flux ratios in the cusp
configurations is unlikely explained by 'simple' perturbers (subhaloes) inside
the lensing galaxy, either modeled by point masses or extended NFW subhaloes.
We considered in our analysis a mass range of 10^5-10^7 Msun for the subhaloes.
We also demonstrate that including the effects of the surrounding mass
distribution, such as other galaxies close to the primary lens, does not change
the results. We conclude that triple images of lensed QSOs do not show any
direct evidence for dark dwarf galaxies such as cold dark matter substructure.
