Mitigation of anomalous APD signals in the CMS ECAL

We describe the observation and mitigation of anomalous, large signals, observed in the barrel part of the CMS Electromagnetic Calorimeter during proton collisions at LHC. Laboratory and beam tests, as well as simulations, have been used to understand their origin. They are ascribed to direct energy deposition by particles in the avalanche photodiodes used for light readout. A reprogramming of the front-end electronics has allowed a majority of these anomalous signals to be identified and rejected at the first (hardware) trigger level with minimum impact on physics performance. Further rejection is performed in the high-level software trigger and offline analyses.