Distributed Denial Of Service in Chi with Basic Auth

Chi is a pragmatic HTTP web framework for the Erlang ecosystem. When Basic Authentication is used in Chi pipelines, the combination of per-request credential validation and resource-intensive request handling can amplify availability risks. Basic Auth requires the server to parse the Authorization header on every request, validate credentials (often via a database or external service), and potentially load or recompute authentication state for each call. Under high concurrency, this can increase CPU and memory pressure, making it easier to exhaust connection pools, scheduler resources, or VM memory.

A DDoS vector emerges when an attacker sends many unauthenticated or poorly authenticated requests that still trigger full Basic Auth validation logic. For example, malformed or repeated Authorization headers can cause repeated parsing and lookup work, and if rate limiting is absent or misconfigured, the service may become unresponsive for legitimate users. In a Chi application that also exposes unauthenticated endpoints, an attacker might probe for endpoints that accept Basic Auth but do not enforce strict input checks, leading to expensive operations tied to credential validation.

Because middleBrick scans the unauthenticated attack surface, it can identify whether authentication-sensitive endpoints are exposed without sufficient rate control or whether the API returns verbose errors that aid reconnaissance. Findings such as missing rate limiting, inefficient credential lookup, or inconsistent authentication enforcement across routes are surfaced with severity and remediation guidance, helping teams understand how DDoS risks intersect with authentication design in Chi.

To reduce DDoS surface when using Basic Auth in Chi, apply rate limiting early in the pipeline, fail fast on malformed credentials, and avoid expensive work before authentication succeeds. The following patterns demonstrate concrete, idiomatic Chi configurations and guards.

1. Rate limiting with token bucket before auth check

Apply a lightweight rate limit as early as possible to prevent resource exhaustion. Use Plug.Throttle or a custom plug that tracks IPs and rejects excess requests before invoking authentication logic.

defmodule MyApp.RateLimit do
  @limit 30
  @interval 60_000

  def call(conn, _opts) do
    case {conn.method, conn.request_path} do
      {"GET", _} ->
        # Lightweight check; plug can be swapped for a distributed token bucket
        Plug.Throttle.check(conn, @limit, @interval)
      _ ->
        {:cont, conn}
    end
  end
end

2. Fail-fast on malformed Authorization header

Validate the presence and format of the Basic Auth header before decoding or querying backends. Return 400 for malformed input to avoid unnecessary work.

defmodule MyApp.BasicAuthValidate do
  import Plug.Conn

  def init(opts), do: opts

  def call(conn, _opts) do
    case get_req_header(conn, "authorization") do
      ["Basic " <> encoded] when byte_size(encoded) > 0 ->
        case Base.url_decode64(encoded) do
          {:ok, decoded} when is_binary(decoded) ->
            # Proceed to credential lookup
            {:cont, conn}
          _ ->
            send_resp(conn, 400, "Malformed Authorization header")
            |> halt()
        end
      _ ->
        # No auth header; let downstream auth plug handle rejection
        {:cont, conn}
    end
  end
end

3. Secure Chi pipeline with guarded authentication and circuit breaker

Structure your pipeline so authentication is checked early for protected routes, and expensive operations are gated behind verified credentials. Use a circuit breaker or backpressure mechanism to protect downstream services during traffic spikes.

defmodule MyAppWeb.Router do
  use MyAppWeb, :router

  pipeline :api do
    plug :accepts, ["json"]
    plug MyApp.RateLimit
    plug MyApp.BasicAuthValidate
    plug MyApp.BasicAuthCheck  # verifies credentials, returns 401 on failure
  end

  pipeline :protected do
    plug :ensure_authenticated
  end

  scope "/api", MyAppWeb do
    pipe_through :api

    get "/public", PublicController, :index

    scope "/secure" do
      pipe_through :protected
      get "/profile", ProfileController, :show
      post "/action", ActionController, :execute
    end
  end
end

4. Use plug-based credential caching to reduce lookup load

Avoid repeated expensive lookups for the same credentials within a short window. Cache validated credentials in process dictionary or via a fast in-memory store with TTL, ensuring cache invalidation on changes.

defmodule MyApp.BasicAuthCache do
  import Plug.Conn

  def init(opts), do: opts

  def call(conn, _opts) do
    case get_req_header(conn, "authorization") do
      ["Basic " <> encoded] ->
        key = {:basic, encoded}
        case Process.get(key) do
          nil ->
            case verify_credentials(encoded) do
              {:ok, user} ->
                Process.put(key, {user, System.monotonic_time()})
                assign(conn, :current_user, user)
              _ ->
                send_resp(conn, 401, "Unauthorized") |> halt()
            end
          {user, _ts} ->
            assign(conn, :current_user, user)
        end
      _ ->
        conn
    end
  end

  defp verify_credentials(encoded) do
    # Replace with actual credential verification
    if Base.url_decode64!(encoded) == "user:pass" do
      {:ok, %{id: 1, username: "user"}}
    else
 :error
    end
  end
end

5. Align with middleBrick findings for authentication and rate limiting

Use middleBrick scans to identify routes where Basic Auth is exposed without adequate rate limiting or where error messages reveal too much about auth internals. Remediation includes tightening pipeline order, adding per-IP and per-user rate limits, standardizing error responses to avoid data leakage, and monitoring for anomalous auth patterns indicative of active probing.