CVE-2026-56819 — Netty: HTTP/2 decompression leaks ByteBuf reference count when the decompressor channel is already closed (Direct memor…

Affected: netty

Netty is a network application framework for development of protocol servers and clients. In versions 4.2.0.Final through 4.2.15.Final and 4.1.0.Final through 4.1.135.Final, 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(...)` calls `decompressor.writeInbound(data.retain())` and does not release the retained buffer on the error path, eventually exhausting direct memory and crashing the JVM. This issue is fixed in versions 4.1.136.Final and 4.2.16.Final.

GHSA-93wv-jw9v-4972 — Netty: HTTP/2 decompression leaks ByteBuf reference count when the decompressor channel is already closed (Direct memor…

Affected: Maven: io.netty:netty-codec-http2

### 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:

```java // 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…

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CVE-2026-56819 — Netty: HTTP/2 decompression leaks ByteBuf reference count when the decompressor channel is already closed (Direct memor…

GHSA-93wv-jw9v-4972 — Netty: HTTP/2 decompression leaks ByteBuf reference count when the decompressor channel is already closed (Direct memor…