Http4s is a Scala interface for HTTP services. Prior to 0.23.35 and 1.0.0-M47, Ember HTTP/2 serializes outbound frames through one unbounded queue consumed by writeLoop. When the peer stops reading, an unauthenticated HTTP/2 client can continue sending PING, SETTINGS, or DATA frames that cause Ember to enqueue acknowledgments or WINDOW_UPDATE frames faster than the writer drains them, exhausting heap memory on a server built with withHttp2. The shared behavior also affects an ember-client connected to a hostile HTTP/2 server, and the patch replaces the unbounded path with bounded, backpressured outbound queues. This issue is fixed in versions 0.23.35 and 1.0.0-M47.

Affected: Maven: org.http4s:http4s-ember-core_2.12, Maven: org.http4s:http4s-ember-core_2.13, Maven: org.http4s:http4s-ember-core_3

Ember's HTTP/2 connection serializes all outgoing frames through a single unbounded queue drained by one writer fiber (`writeLoop`). When the write side stalls, any frames the connection keeps producing accumulate in that queue without limit. The peer can drive this cheaply because the connection emits a control frame in response to inbound frames it does not flow-control: one `PING` ACK per `PING`, one `SETTINGS` ACK per `SETTINGS`, and a `WINDOW_UPDATE` per inbound DATA. A single unauthenticated connection can therefore exhaust heap and OOM the process.

This affects an ember server (malicious client) and an ember client (malicious/compromised server that floods the client and stops reading its ACKs).

## Impact

Unauthenticated remote denial of service (OOM) against any ember server built `.withHttp2`, from a single connection, at negligible attacker cost (tiny control frames). Also affects an ember client talking to a hostile HTTP/2 server.

## Preconditions

- Ember server or client built `.withHttp2`, speaking to a hostile or compromised peer.

## Workarounds

- Disable HTTP/2 (do not call `.withHttp2`).