GHSA-93wv-jw9v-4972 Netty: HTTP/2 decompression leaks ByteBuf reference count when the decompressor channel is already closed (Direct memory leak / OOM DoS)
SummaryA 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.DetailsIn `codec-http2/src/main/java/io/netty/handler/codec/http2/DelegatingDecompressorFrameListener.java`,
`Http2Decompressor.decompress(...)` does: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(...)`
— the `decompressor.writeInbound(data.retain())` call (line ~433) and its `catch (Throwable t)`
block (line ~451), which lacks a `data.release()` rollback.**Suggested fix**: track whether `writeInbound` succeeded and roll back the extra `retain()` only when
the data never entered the pipeline:| Case | writeSucceeded | catch action | Reason |
|------|:---:|---|---|
| `ensureOpen()` throws (this bug) | `false` | `data.release()` | data never entered pipeline |
| handler throws internally | `true` | no release | decoder `finally` already released |
| `finish()` throws | `true` | no release | `writeInbound` already succeeded |PoCReproduced against the official, unmodified `netty-codec-http2-4.2.15.Final.jar` from Maven Central,
using real netty classes and measuring `ByteBuf.refCnt()` directly (the leaking logic is not mocked).Reproduction steps:Download the official artifacts and their dependencies from Maven Central (version `4.2.15.Final`):
`netty-common`, `netty-buffer`, `netty-transport`, `netty-resolver`, `netty-handler`,
`netty-codec-base`, `netty-codec`, `netty-codec-http`, `netty-codec-http2`,
`netty-codec-compression`.Build a real `Http2Decompressor` wrapping a real gzip decoder `EmbeddedChannel`
(`ZlibCodecFactory.newZlibDecoder(ZlibWrapper.GZIP)`).Close the internal decompressor channel (equivalent to the end state of
`cleanup()` / `finishAndReleaseAll()`).Encode a real gzip `DATA` payload with `ZlibCodecFactory.newZlibEncoder(GZIP)` (`refCnt = 1`).Call `decompress(...)` on the closed channel.Observe: `writeInbound(...)` throws `ClosedChannelException` at its `ensureOpen()` entry
(`EmbeddedChannel.java:360`), reached from `DelegatingDecompressorFrameListener.java:433`;
`data.refCnt()` is now `2`.Release once as the frame reader would; `refCnt` stays at `1` (`release()` returns `false`) → leaked.Observed reference-count trace:Observed exception stack (confirms the leak point):Two notes on the harness (they do not affect the leak mechanism):The internal channel is closed directly via `close()` rather than through `cleanup()`. The end
state is identical (channel closed → `writeInbound` throws at `ensureOpen()`); the bug depends on
"channel closed → retain not rolled back", not on *how* the channel was closed.In the isolated harness the rethrown `StreamException`'s root cause shows as `NullPointerException`
because the harness does not initialise an `Http2LocalFlowController` (a secondary exception
reported during channel close). The leak is already sealed at the `ClosedChannelException` thrown
by `writeInbound`'s `ensureOpen()` (line 360); in a real server with the flow controller
initialised, the triggering exception is the `ClosedChannelException` itself.A complete self-contained PoC (`Verify02DecompressLeak.java`, ~150 lines, no test framework) plus the
exact `javac` / `java` commands can be attached on request.Impact**Vulnerability type**: uncontrolled resource consumption / memory leak (CWE-401), leading to
denial of service. Each crafted `DATA` frame leaks one (typically direct/off-heap) `ByteBuf`.**Who is impacted**: any server (or client) that enables HTTP/2 content decompression by installing
`DelegatingDecompressorFrameListener` in its HTTP/2 pipeline.**Attacker requirements**: remote, unauthenticated. The attacker only needs to send HTTP/2 `DATA`
frames for a stream whose decompressor has been cleaned up (e.g. continue sending `DATA` after
`END_STREAM`). No special server configuration beyond decompression being enabled.**Result**: sustained triggering over a long-lived connection exhausts direct memory and crashes
the JVM with `OutOfMemoryError`.
CVSS v3.1 score: 7.5