gRPC Erlang package has unbounded request body accumulation in `read_full_body/3`

Summary

'Elixir.GRPC.Server.Adapters.Cowboy.Handler':read_full_body/3 accumulates every received chunk into a single growing binary with no size cap. When the client omits the grpc-timeout header, the read timeout resolves to :infinity, allowing a slow-trickle attacker to hold the connection open indefinitely while memory grows. A single unauthenticated connection is sufficient to exhaust BEAM memory and crash the node.

Details

The read loop in lib/grpc/server/adapters/cowboy/handler.ex calls :cowboy_req.read_body/2 in a recursive loop, concatenating each chunk: body <> data. There is no running-total check and no configurable maximum body size. As the loop drains the receive buffer, cowboy issues fresh HTTP/2 WINDOW_UPDATE frames, so the client can keep pushing data indefinitely.

The grpc-timeout header is attacker-supplied and optional. When absent, timeout_left_opt(nil) returns :infinity, so the per-chunk read also has no deadline. The two missing controls compound: a fast client can blast multi-gigabyte payloads directly into memory; a slow client can trickle data forever.

PoC

1. Start any grpc server exposing a unary RPC (no special configuration required).
2. Open an HTTP/2 connection and send a POST to any unary RPC path with Content-Type: application/grpc+proto — omit the grpc-timeout header.
3. Stream a large body (e.g. 1 GiB) in chunks without sending the final END_STREAM flag immediately.
4. Observe BEAM memory growing proportionally to uploaded data with no server-side cap.

Impact

Affects grpc ≥ 0.3.1. No authentication, no special configuration, and no specific RPC method required, the unbounded read is on the default unary ingress path.

References

• Introduction commit: https://github.com/elixir-grpc/grpc/commit/d1abe70a6cad6dac4a3f8235d883d7c896989560
• Patch commit: https://github.com/elixir-grpc/grpc/commit/49e18c3ec6bb9afe2f712caad3dbab5c56a68a00