From f6d48c90f93b3d3a0560074d2cd9b3a9be891a64 Mon Sep 17 00:00:00 2001
From: Shane Freeder <theboyetronic@gmail.com>
Date: Wed, 8 Apr 2026 20:41:09 +0100
Subject: [PATCH] reduce clientbound compression limits

---
 .../proxy/connection/MinecraftConnection.java |  4 +++-
 .../netty/MinecraftCompressDecoder.java       | 23 ++++++++++++++-----
 2 files changed, 20 insertions(+), 7 deletions(-)

diff --git a/proxy/src/main/java/com/velocitypowered/proxy/connection/MinecraftConnection.java b/proxy/src/main/java/com/velocitypowered/proxy/connection/MinecraftConnection.java
index f7de55e0f..151cbd303 100644
--- a/proxy/src/main/java/com/velocitypowered/proxy/connection/MinecraftConnection.java
+++ b/proxy/src/main/java/com/velocitypowered/proxy/connection/MinecraftConnection.java
@@ -38,6 +38,7 @@ import com.velocitypowered.proxy.connection.client.InitialLoginSessionHandler;
 import com.velocitypowered.proxy.connection.client.StatusSessionHandler;
 import com.velocitypowered.proxy.network.Connections;
 import com.velocitypowered.proxy.protocol.MinecraftPacket;
+import com.velocitypowered.proxy.protocol.ProtocolUtils;
 import com.velocitypowered.proxy.protocol.StateRegistry;
 import com.velocitypowered.proxy.protocol.VelocityConnectionEvent;
 import com.velocitypowered.proxy.protocol.netty.MinecraftCipherDecoder;
@@ -544,9 +545,10 @@ public class MinecraftConnection extends ChannelInboundHandlerAdapter {
       } else {
         int level = server.getConfiguration().getCompressionLevel();
         VelocityCompressor compressor = Natives.compress.get().create(level);
+        final MinecraftDecoder minecraftDecoder = (MinecraftDecoder) channel.pipeline().get(MINECRAFT_DECODER);
 
         encoder = new MinecraftCompressorAndLengthEncoder(threshold, compressor);
-        decoder = new MinecraftCompressDecoder(threshold, compressor);
+        decoder = new MinecraftCompressDecoder(threshold, compressor, minecraftDecoder.getDirection());
 
         channel.pipeline().remove(FRAME_ENCODER);
         channel.pipeline().addBefore(MINECRAFT_DECODER, COMPRESSION_DECODER, decoder);
diff --git a/proxy/src/main/java/com/velocitypowered/proxy/protocol/netty/MinecraftCompressDecoder.java b/proxy/src/main/java/com/velocitypowered/proxy/protocol/netty/MinecraftCompressDecoder.java
index 1fe38e50e..9bd040efa 100644
--- a/proxy/src/main/java/com/velocitypowered/proxy/protocol/netty/MinecraftCompressDecoder.java
+++ b/proxy/src/main/java/com/velocitypowered/proxy/protocol/netty/MinecraftCompressDecoder.java
@@ -33,20 +33,26 @@ import java.util.List;
  */
 public class MinecraftCompressDecoder extends MessageToMessageDecoder<ByteBuf> {
 
+  private static final int SERVERBOUND_MAXIMUM_UNCOMPRESSED_SIZE = 2 * 1024 * 1024; // 2MiB
   private static final int VANILLA_MAXIMUM_UNCOMPRESSED_SIZE = 8 * 1024 * 1024; // 8MiB
   private static final int HARD_MAXIMUM_UNCOMPRESSED_SIZE = 128 * 1024 * 1024; // 128MiB
 
-  private static final int UNCOMPRESSED_CAP =
+  private static final int CLIENTBOUND_UNCOMPRESSED_CAP =
       Boolean.getBoolean("velocity.increased-compression-cap")
           ? HARD_MAXIMUM_UNCOMPRESSED_SIZE : VANILLA_MAXIMUM_UNCOMPRESSED_SIZE;
+  private static final int SERVERBOUND_UNCOMPRESSED_CAP =
+          Boolean.getBoolean("velocity.increased-compression-cap")
+                  ? HARD_MAXIMUM_UNCOMPRESSED_SIZE : SERVERBOUND_MAXIMUM_UNCOMPRESSED_SIZE;
   private static final boolean SKIP_COMPRESSION_VALIDATION = Boolean.getBoolean("velocity.skip-uncompressed-packet-size-validation");
+  private final ProtocolUtils.Direction direction;
 
   private int threshold;
   private final VelocityCompressor compressor;
 
-  public MinecraftCompressDecoder(int threshold, VelocityCompressor compressor) {
+  public MinecraftCompressDecoder(int threshold, VelocityCompressor compressor, ProtocolUtils.Direction direction) {
     this.threshold = threshold;
     this.compressor = compressor;
+    this.direction = direction;
   }
 
   @Override
@@ -65,10 +71,15 @@ public class MinecraftCompressDecoder extends MessageToMessageDecoder<ByteBuf> {
 
     checkFrame(claimedUncompressedSize >= threshold, "Uncompressed size %s is less than"
         + " threshold %s", claimedUncompressedSize, threshold);
-    checkFrame(claimedUncompressedSize <= UNCOMPRESSED_CAP,
-        "Uncompressed size %s exceeds hard threshold of %s", claimedUncompressedSize,
-        UNCOMPRESSED_CAP);
-
+    if (direction == ProtocolUtils.Direction.CLIENTBOUND) {
+      checkFrame(claimedUncompressedSize <= CLIENTBOUND_UNCOMPRESSED_CAP,
+              "Uncompressed size %s exceeds hard threshold of %s", claimedUncompressedSize,
+              CLIENTBOUND_UNCOMPRESSED_CAP);
+    } else {
+      checkFrame(claimedUncompressedSize <= SERVERBOUND_UNCOMPRESSED_CAP,
+              "Uncompressed size %s exceeds hard threshold of %s", claimedUncompressedSize,
+              SERVERBOUND_UNCOMPRESSED_CAP);
+    }
     ByteBuf compatibleIn = ensureCompatible(ctx.alloc(), compressor, in);
     ByteBuf uncompressed = preferredBuffer(ctx.alloc(), compressor, claimedUncompressedSize);
     try {