  Warm Dark Matter (WDM) has been invoked to resolve apparent conflicts of Cold
Dark Matter (CDM) models with observations on subgalactic scales. In this work
we provide a new and independent lower limit for the WDM particle mass (e.g.
sterile neutrino) through the analysis of image fluxes in gravitationally
lensed QSOs.
  Starting from a theoretical unperturbed cusp configuration we analyze the
effects of intergalactic haloes in modifying the fluxes of QSO multiple images,
giving rise to the so-called anomalous flux ratio. We found that the global
effect of such haloes strongly depends on their mass/abundance ratio and it is
maximized for haloes in the mass range $10^6-10^8 \Msun$.
  This result opens up a new possibility to constrain CDM predictions on small
scales and test different warm candidates, since free streaming of warm dark
matter particles can considerably dampen the matter power spectrum in this mass
range. As a consequence, while a ($\Lambda$)CDM model is able to produce flux
anomalies at a level similar to those observed, a WDM model, with an
insufficiently massive particle, fails to reproduce the observational
evidences.
  Our analysis suggests a lower limit of a few keV ($m_{\nu} \sim 10$) for the
mass of warm dark matter candidates in the form of a sterile neutrino. This
result makes sterile neutrino Warm Dark Matter less attractive as an
alternative to Cold Dark Matter, in good agreement with previous findings from
Lyman-$\alpha$ forest and Cosmic Microwave Background analysis.
