  The injection of secondary particles produced by Dark Matter (DM)
annihilation at redshift 100<z<1000 affects the process of recombination,
leaving an imprint on Cosmic Microwave Background (CMB) anisotropies. Here we
provide a new assessment of the constraints set by CMB data on the mass and
self-annihilation cross-section of DM particles. Our new analysis includes the
most recent WMAP (7-year) and ACT data, as well as an improved treatment of the
time-dependent coupling between the DM annihilation energy with the thermal
gas. We show in particular that the improved measurement of the polarization
signal places already stringent constraints on light DM particles, ruling out
'thermal' WIMPs with mass less then about 10 GeV.
