Departing from the classical Burke-Schumann theory for diffusion flames, a reduced model for the description of the evolving wave front in forward smoldering is derived. The model is then used to describe the fingering instability induced by the increase in porosity, which results from the solid fuel depletion. The cusp-leading nature of the fingering pattern is revealed, which is opposite to the cusp-trailing structure typical of premixed gas cellular/wrinkled flames. (c) 2008 The Combustion Institute. Published by Elsevier Inc. All rights reserved.