Topological defects, such as dislocations in crystals, are stable states with positive energy. Their stability stems from breaking symmetry and continuity in the medium. Cracks also favor asymmetry in the surfaces they leave behind; even in wholly amorphous materials brittle fracture surfaces are commonly divided into smooth facets by discontinuous steps. Step formation by the leading edge of the crack -the crack front - is a 3D process in which the front deforms both parallel (in-plane) and perpendicular to the surface (out-of-plane). Here we show, by integrating real time imaging of propagating crack fronts with surface measurements, that steps are topological defects of the crack front. Steps increase energy dissipation by adding surface area. Their propagation is determined by reflection symmetry breaking, while step propagation angles relative to the local crack front directions remains constant, even when front orientation changes considerably. Finally, we show how in-plane and out-of-plane deformations of the crack front are linked.