Server uses block reach distance of 8 whereas client uses lower reach distance

The following is based on a decompiled version of Minecraft 1.10 using MCP 9.30.

The bug

The client uses the reach distance defined by the method net.minecraft.client.multiplayer.PlayerControllerMP.getBlockReachDistance(), which is in 1.10

• Creative: 5
• Other gamemodes: 4.5

The method net.minecraft.network.NetHandlerPlayServer.processRightClickBlock(CPacketPlayerTryUseItemOnBlock) uses 8 however. This means a modified client can exploit that and in case a gamemode with a higher reach distance is introduced, trying to interact with blocks 8 or more meters away will fail.

Note: Additionally this method does not test if the player can really see the position at which he is pretending to click. That means a modified client would be able to activate for example buttons through walls.