Http4s: Ember HTTP/2 buffers a frame's declared payload before checking SETTINGS_MAX_FRAME_SIZE

Summary

An unauthenticated peer can make Ember's HTTP/2 read loop hold 16 MiB of a single frame in memory on a connection where Ember advertised a 16 KiB limit. The declared length is readable from the frame's first 9 bytes, but it is not compared against SETTINGS_MAX_FRAME_SIZE until the whole payload has been read into a contiguous buffer. That is 1024x amplification per connection, bounded by the protocol's 24-bit length field rather than by any setting, and at the default maxConnections of 1024 it puts about 16 GiB of heap in reach.

Details

H2Frame.RawFrame.fromByteVector (ember-core/.../h2/H2Frame.scala:66-69) reads the length, then refuses to produce a frame until 9 + length bytes are present:

val length = (bv(2) & 0xff) | ((bv(1) & 0xff) << 8) | ((bv(0) & 0xff) << 16)
if (bv.length >= 9 + length) {

readNextFrame in H2Connection.readLoop (H2Connection.scala:246-266) reads that None as "need more" and concatenates further socket reads into one accumulator until it is satisfied.

The only check of an inbound frame against the connection's own SETTINGS_MAX_FRAME_SIZE is in processFrame at H2Connection.scala:534, which cannot run until the frame is assembled. So the value that condemns the frame sits in the accumulator from byte 9 onward and is never consulted. A peer that declares 16777215 and dribbles the payload without finishing gets the same buffering with no reaction at all, since an incomplete frame never reaches processFrame.

RFC 9113 4.2 requires treating an oversized frame as a connection error and explicitly permits responding without reading the rest of the payload.

Impact

Uncontrolled resource consumption leading to unauthenticated remote denial of service by memory exhaustion.

• ember-server built .withHttp2: any peer able to open an HTTP/2 connection can trigger it. No authentication and no valid request, since the frame is rejected before it is associated with a stream, and the incomplete variant is never rejected.
• ember-client built .withHttp2 against a hostile origin: readLoop is shared, so the client side is symmetric. Reasoned from the shared code path, not reproduced.
• Not affected: HTTP/2 off, which is the default on both builders.

The overshoot is not configurable. It comes from the protocol's length field, not from Ember's limit, so tuning SETTINGS_MAX_FRAME_SIZE down does not reduce exposure, and withIdleTimeout does not apply because the read loop is making steady progress. Workarounds: leave HTTP/2 off, or terminate it at a proxy that enforces frame size and speak HTTP/1.1 to Ember.