The Wither effect can harm undead mobs if a Wither accidently hits an undead mob with a Wither Skull, though it should not.
Bing's definition of wither: to shrivel or dry up as part of the process of dying, or make something, especially a plant or part of a plant, shrivel in this way.
Steps to Reproduce:
1. Make a new Creative world.
2. Spawn a group of pigs (or any living mobs).
3. Spawn a few of any undead mobs within the group of pigs (make sure it's night if using Skeletons or Zombies)
4. Spawn a Wither a small distance away, but not too close.
5. When the Wither eventually hits one of the undead mobs directly, notice it takes damage from the Wither effect.
I'm unsure if this is intended, or if they forgot to make undead mobs immune to the Wither effect.