Movement exploit involving no collision checking while in a block

Video demonstrating: https://www.youtube.com/watch?v=VBFNyfP2unY

Relevant code: https://gist.github.com/Spottedleaf/f0c3df24da75f07901ab31c3cc1669ca

PlayerConnection.java, spigot mappings, vanilla code + server

Important lines: 64, 95, 97

 

The server will only teleport the player back during a collision check if the player was originally not in a block. However, using a door, you can get yourself into a block (or with sand/gravel, and anything else I've forgotten). Then from there the server will not collision check you and you can roam freely under the ground, or perhaps through a wall.

 

I will post a fix here when I have one, although it's likely going to be a temporary fix.

 

Side note: This can also be a cause for players clipping through chunks after teleporting (this is how I found this particular exploit actually), since the client would fall until a chunk loaded in for it (so teleporting into a door or block could cause the player to clip through the chunk).