When I connect two rigid bodies (with sphere colliders) using a springjoint they do not react to each others colliders - they can pass through each other. Is this normal behavior or is it a bug?
This is normal behaviour, all joints will not collide with the object directly connected to it. This is important for providing stable physics simulations.
In the case of a spring joint it sometimes makes sense to disable this.
We will provide an option to enable collision between jointed rigidbodies in a future release.
To work around it for now, just create another kinematic rigidbody with no colliders attached and attach the spring joint to that instead.