Strictly limit the hostname size and limit it to ASCII characters only

proxy/src/main/java/com/velocitypowered/proxy/protocol/ProtocolUtils.java

@@ -102,7 +102,7 @@ public enum ProtocolUtils {
   }
 
   /**
-   * Reads a VarInt length-prefixed string from the {@code buf}, making sure to not go over
+   * Reads a VarInt length-prefixed UTF-8 string from the {@code buf}, making sure to not go over
    * {@code cap} size.
    * @param buf the buffer to read from
    * @param cap the maximum size of the string, in UTF-8 character length
@@ -113,6 +113,24 @@ public enum ProtocolUtils {
     return readString(buf, cap, length);
   }
 
+  /**
+   * Reads a VarInt length-prefixed ASCII string from the {@code buf}, making sure to not go over
+   * {@code cap} size. This method is specialized for select parts of the Minecraft protocol where
+   * ASCII characters are guaranteed to be used.
+   *
+   * @param buf the buffer to read from
+   * @param cap the maximum size of the string, in UTF-8 character length
+   * @return the decoded string
+   */
+  public static String readAsciiString(ByteBuf buf, int cap) {
+    int length = readVarInt(buf);
+    checkFrame(length >= 0, "Got a negative-length string (%s)", length);
+    checkFrame(length <= cap, "Bad string size (got %s, maximum is %s)", length, cap);
+    String str = buf.toString(buf.readerIndex(), length, StandardCharsets.US_ASCII);
+    buf.skipBytes(length);
+    return str;
+  }
+
   private static String readString(ByteBuf buf, int cap, int length) {
     checkFrame(length >= 0, "Got a negative-length string (%s)", length);
     // `cap` is interpreted as a UTF-8 character length. To cover the full Unicode plane, we must

proxy/src/main/java/com/velocitypowered/proxy/protocol/packet/Handshake.java

@@ -1,13 +1,19 @@
 package com.velocitypowered.proxy.protocol.packet;
 
+import static com.velocitypowered.proxy.connection.forge.legacy.LegacyForgeConstants.HANDSHAKE_HOSTNAME_TOKEN;
+
 import com.velocitypowered.api.network.ProtocolVersion;
 import com.velocitypowered.proxy.connection.MinecraftSessionHandler;
+import com.velocitypowered.proxy.connection.forge.legacy.LegacyForgeConstants;
 import com.velocitypowered.proxy.protocol.MinecraftPacket;
 import com.velocitypowered.proxy.protocol.ProtocolUtils;
 import io.netty.buffer.ByteBuf;
 
 public class Handshake implements MinecraftPacket {
 
+  // This size was chosen to ensure Forge clients can still connect even with very long hostnames.
+  // While DNS technically allows any character to be used, in practice ASCII is used.
+  private static final int MAXIMUM_HOSTNAME_LENGTH = 255 + HANDSHAKE_HOSTNAME_TOKEN.length() + 1;
   private ProtocolVersion protocolVersion;
   private String serverAddress = "";
   private int port;
@@ -59,7 +65,7 @@ public class Handshake implements MinecraftPacket {
   public void decode(ByteBuf buf, ProtocolUtils.Direction direction, ProtocolVersion ignored) {
     int realProtocolVersion = ProtocolUtils.readVarInt(buf);
     this.protocolVersion = ProtocolVersion.getProtocolVersion(realProtocolVersion);
-    this.serverAddress = ProtocolUtils.readString(buf);
+    this.serverAddress = ProtocolUtils.readAsciiString(buf, MAXIMUM_HOSTNAME_LENGTH);
     this.port = buf.readUnsignedShort();
     this.nextStatus = ProtocolUtils.readVarInt(buf);
   }