Inspired by experiments on the instability of confined interfacial cracks, we construct a minimal
mathematical model based on symmetry arguments that can reproduce the form of the crack front in a
confined domain. We show that the model can be interpreted in terms of the buckling and postbuckling response of a compressed elastica with a nonuniform bending stiffness that is adhered to a
linearly elastic substrate. The model has three parameters that allow us to capture the primary
wavelength associated with the onset of an undulatory instability of a straight crack front, as well as the
finger amplitudes and finger widths in the nonlinear development of the instability. We determine these
parameters using an optimization procedure that minimizes the square error between the computed
profile and experimental observations. The results of this procedure yield numerical solutions that agree
well with the finger profiles experimentally observed in films of different thicknesses. Our approach
shows the efficacy of simple models based on symmetry in explaining interfacial instabilities governed
by different physical mechanisms.