If the decompressor channel has been closed or cleaned up, the writeInbound() call fails with a ClosedChannelException. Because the code does not perform a rollback of the reference count increment in this error path, the buffer is never released. An attacker can exploit this by sending DATA frames for streams that have already reached an END_STREAM state or been cleaned up. Sustained exploitation causes the accumulation of unreferenced direct memory objects, leading to an OutOfMemoryError (DoS) and application crash. This affects users of netty-codec-http2 across the 4.1.x and 4.2.x branches.

Attack Chain

1. Attacker establishes a legitimate, long-lived HTTP/2 connection to a vulnerable server.
2. Attacker initiates a stream and sends data frames to trigger standard decompressor processing.
3. Attacker sends a final frame to signal END_STREAM, forcing the server to trigger onStreamRemoved and Http2Decompressor.cleanup().
4. Attacker continues to send additional DATA frames associated with the now-closed stream.
5. The DelegatingDecompressorFrameListener calls decompressor.writeInbound(data.retain()).
6. The EmbeddedChannel.ensureOpen() method throws a ClosedChannelException because the channel is closed.
7. The catch block in Http2Decompressor fails to call data.release() on the leaked buffer.
8. The direct memory is leaked; repetition of this process exhausts the heap and crashes the service.

Impact

The vulnerability allows an unauthenticated remote attacker to cause a denial-of-service condition by crashing the JVM. This affects any application utilizing the DelegatingDecompressorFrameListener component within an HTTP/2 pipeline, which is a common pattern for high-performance network services. Repeated exploitation leads to direct memory exhaustion, which is particularly critical in containerized environments with memory limits where the OOM error occurs rapidly.

Recommendation

1. Upgrade to the latest patched version of netty-codec-http2 provided by the Netty Project to remediate CVE-2026-56819.
2. Implement an application-level rate limit on HTTP/2 streams to detect and disconnect peers that continue to send data after a stream has been closed, which serves as a mitigation until patching can occur.
3. If patching is not immediately feasible, inspect the DelegatingDecompressorFrameListener implementation and manually apply the rollback logic as documented in the GHSA-93wv-jw9v-4972 advisory to ensure the reference count is decremented on failure.