The injection of secondary particles produced by Dark Matter (DM) annihilation at redshift100 <∼ z <∼ 1000 affects the process of recombination, leaving an imprint on Cosmic MicrowaveBackground (CMB) anisotropies. Here we provide a new assessment of the constraints set by CMBdata on the mass and self-annihilation cross-section of DM particles. Our new analysis includes themost recent WMAP (7-year) and ACT data, as well as an improved treatment of the time-dependentcoupling between the DM annihilation energy with the thermal gas. We show in particular that theimproved measurement of the polarization signal places already stringent constraints on light DMparticles, ruling out ‘thermal’ WIMPs with mass mχ <∼ 10 GeV