Affected: netty, io.netty netty-codec, io.netty netty-codec-compression

Netty is an asynchronous, event-driven network application framework. Prior to 4.2.13.Final and 4.1.133.Final, 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. This vulnerability is fixed in 4.2.13.Final and 4.1.133.Final.

Affected: Maven: io.netty:netty-codec-compression, Maven: io.netty:netty-codec

### 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() {…