GHSA-93wv-jw9v-4972: Netty: HTTP/2 decompression leaks ByteBuf reference count when the decompressor channel is already closed (Direct memory leak / OOM DoS)

Summary A remote, unauthenticated peer can leak one direct ByteBuf per HTTP/2 DATA frame in applications that enable HTTP/2 content decompression via DelegatingDecompressorFrameListener. When a DATA frame is processed for a stream whose decompressor has already been closed, Http2Decompressor.decompress(...) retains the frame buffer but never releases it on the error path, so its reference count never returns to zero. Repeating this over a long-lived HTTP/2 connection exhausts direct memory and crashes the JVM with OutOfMemoryError — a denial of service. Details In codec-http2/src/main/java/io/netty/handler/codec/http2/DelegatingDecompressorFrameListener.java, Http2Decompressor.decompress(...) does: // around line 433 decompressor.writeInbound(data.retain()); The argument data.retain() is evaluated before writeInbound(...) executes, incrementing the buffer's reference count (refCnt: 1 -> 2). The very first statement of EmbeddedChannel.writeInbound(...) is ensureOpen() (EmbeddedChannel.java:360), which throws ClosedChannelException when the decompressor's internal EmbeddedChannel has already been closed. When that happens: - the DATA payload has been retain()ed but never entered the pipeline, so the decoder's finally { release() } never runs; - the surrounding catch (Throwable t) block in decompress(...) (around line 451) does not release the extra reference; - the input buffer therefore can never reach refCnt 0, and its (typically direct) memory is leaked. The decompressor channel is closed on a reachable path: Http2Connection onStreamRemoved → Http2Decompressor.cleanup() → EmbeddedChannel.finishAndReleaseAll() (DelegatingDecompressorFrameListener.java:125-133 and 418-420). A peer that sends DATA frames for a stream whose decompressor has already been cleaned up (e.g. continuing to send DATA after END_STREAM / stream removal) thus leaks one direct ByteBuf per frame. Affected code: DelegatingDecompressorFrameListener.java, method Http2Decompressor.decompress(