Currently, the packets MessageDismount, MessageHurtMultipart, and MessageMountPlayer can be used maliciously via exploitable client trust.
The root of the exploitation with these packets is that clients are able to send these packets to the server and there are not many internal checks done inside the handling of these packets, trusting the client to do these checks.
MessageDismount - Clients can arbitrarily tell the server to dismount any CreatureEntity from the entity it is riding.

MessageHurtMultipart - Clients can arbitrarily tell the server to damage any IHurtableMultipart entity with any amount of damage.

MessageMountPlayer - Clients can arbitrarily tell the server to mount any CreatureEntity to another entity.

Will fix the Dismount and mount ones - as for the HurtMultipart, all that does is sync the animation frames for hurtTime with the server, it doesn't actually damage the entity.