I don't think it's overcosted, but I think the attack should be higher than 1 on a 5 cost weapon, regardless of effect. A 5 mana weapon that doesn't threaten face damage and your opponent can easily play around with positioning. It seems bad to me.
You don't get to choose the minion it attacks. If they have 3 minions, you are not sure which of the other 2 it will attack. If would be better if it attacked both minions on either side of it.
So when I said this is Betrayal x3. I was wrong. it's not even that good. Poison or not.
Let's say you are facing against 3 minions and face tank a giant and take 8 damage. The giant then attacks 1 of the other 2 minions. You can't control it. What if you NEED for the Giant to kill the other Giant and it doesnt? You just took 8 damage, didnt remove that Giant and now have 2 giants swinging back at you next turn.
No, this is not under-rated. This is simply terrible. If it attacked all adjacent minions, I would be with you.
Edit - I realized my mistake and admitted it later. It's not a good weapon, but not terrible either. I would call it situational. Maybe a 1 of if the Meta turns into greedy control.