From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
From: Shane Freeder <theboyetronic@gmail.com>
Date: Sun, 18 Apr 2021 21:27:01 +0100
Subject: [PATCH] Validate bungee forwarded hostname

Seriously, fix your firewalls. -.-

diff --git a/src/main/java/net/minecraft/server/network/HandshakeListener.java b/src/main/java/net/minecraft/server/network/HandshakeListener.java
index b97d289afdff81d9959e238639f4e3e186f8e9c8..423696df365ceb20b673a87714b753d7b3b7a3af 100644
--- a/src/main/java/net/minecraft/server/network/HandshakeListener.java
+++ b/src/main/java/net/minecraft/server/network/HandshakeListener.java
@@ -1,5 +1,8 @@
 package net.minecraft.server.network;
 
+import com.google.common.net.InetAddresses;
+import com.google.common.net.InternetDomainName;
+
 import net.minecraft.SharedConstants;
 import net.minecraft.network.EnumProtocol;
 import net.minecraft.network.NetworkManager;
@@ -26,6 +29,7 @@ public class HandshakeListener implements PacketHandshakingInListener {
     private static final IChatBaseComponent a = new ChatComponentText("Ignoring status request");
     private final MinecraftServer b;
     private final NetworkManager c; final NetworkManager getNetworkManager() { return this.c; } // Paper - OBFHELPER
+    private static final boolean BYPASS_HOSTCHECK = Boolean.getBoolean("Paper.bypassHostCheck"); // Paper
 
     public HandshakeListener(MinecraftServer minecraftserver, NetworkManager networkmanager) {
         this.b = minecraftserver;
@@ -114,6 +118,14 @@ public class HandshakeListener implements PacketHandshakingInListener {
                 //if (org.spigotmc.SpigotConfig.bungee) { // Paper - comment out, we check above!
                         String[] split = packethandshakinginsetprotocol.hostname.split("\00");
                         if ( split.length == 3 || split.length == 4 ) {
+                            // Paper start
+                            if (!BYPASS_HOSTCHECK && !validate(split[1])) {
+                                final ChatMessage message = new ChatMessage("Invalid hostname");
+                                this.getNetworkManager().sendPacket(new PacketLoginOutDisconnect(message));
+                                this.getNetworkManager().close(message);
+                                return;
+                            }
+                            // Paper end
                             packethandshakinginsetprotocol.hostname = split[0];
                             c.socketAddress = new java.net.InetSocketAddress(split[1], ((java.net.InetSocketAddress) c.getSocketAddress()).getPort());
                             c.spoofedUUID = com.mojang.util.UUIDTypeAdapter.fromString( split[2] );
@@ -158,4 +170,11 @@ public class HandshakeListener implements PacketHandshakingInListener {
     public NetworkManager a() {
         return this.c;
     }
+
+    // Paper start - https://stackoverflow.com/questions/9954140/check-if-a-string-is-a-hostname-or-an-ip-address-in-java
+    public static boolean validate(final String hostname) {
+        //noinspection UnstableApiUsage
+        return InetAddresses.isUriInetAddress(hostname);
+    }
+    // Paper end
 }