In your vertex shader, you are computing the vertex normal with
frag_normal = normalize(WorldMatrix * vec4(vert_normal, 1.0));
In homogenous coordinates, the last "w" scalar value set to 1.0 means that vert_normal is a point. As it is a mere unpositioned vector, i think using 0.0 should fix your bug.
Moreover, normalizing the interpolation variable frag_normal (set as "smooth out" and "smooth in" by default) will lead to incorrect normal vectors as they won't stay normed during their interpolation across vertices. You should normalize it in the fragment shader.