Hacked client "Kronos" crash vanilla servers

People can use certain hacked clients to crash vanilla servers. It is very hard to run a vanilla server when hackers can already do all the stuff they can't on bukkit but now crash the server and make it unplayable.

A bug or exploit going on with vanilla servers by crashing it.

Player joins a *vanilla server and has a lovely day.*
A player joins the server and plays.

Some players who join the server crash the server.
Some players crash the server after a while on the server or when they join. (Hacks)

Steps to Reproduce:
1. Download a hacked client (Kronos)
2. Install the hacked client.
3. Join a vanila server.
4. Find the server crasher and click it.