Request Smuggling in Buffalo with Basic Auth

Request smuggling occurs when an intermediary (such as a load balancer or reverse proxy) processes HTTP requests differently from the origin server, allowing attackers to smuggle requests across security boundaries. In Buffalo, this can happen when requests that include Basic Auth credentials are parsed inconsistently between the front-end and back-end. When a request contains both a smuggler-crafted header sequence and valid Basic Auth credentials, the front-end may treat the request as two separate logical requests while the backend processes them as one combined message. This mismatch can cause one request to be associated with authenticated context (Basic Auth) while another is not, bypassing intended access controls.

Basic Auth itself does not cause smuggling, but its presence can amplify the impact. Because credentials are transmitted in the Authorization header, a smuggled request that successfully reaches the backend may be processed with elevated privileges. For example, an attacker may smuggle a request so that an authenticated request to an admin endpoint is interpreted as two requests: one benign request that passes authentication checks and a second request that executes an unintended operation under the same authentication context. Buffalo applications that parse headers manually or rely on non-strict HTTP message handling are particularly susceptible when Basic Auth is used, as header ordering and whitespace differences can trigger parsing divergence.

Real-world attack patterns mirror known CVEs such as CVE-2023-23969, where malformed header sequences and ambiguous message parsing enabled cross-request request smuggling. In Buffalo, using the basic_auth module with strict header validation is essential. The framework must ensure that each incoming request results in a single, unambiguous parsed message, and that the Authorization header is never interpreted differently across layers. Without strict validation, an API endpoint scanned by tools like middleBrick may reveal inconsistent parsing behavior in unauthenticated scans, highlighting risk areas tied to authentication and input validation checks.

Remediation focuses on preventing header ambiguity and ensuring consistent parsing of requests that include Basic Auth. Buffalo provides built-in parsing for the Authorization header when the basic_auth plug is used. Rely on this plug rather than manual header extraction to avoid discrepancies between expected and actual request boundaries.

Ensure your router and pipeline are configured to reject requests with ambiguous or duplicated header names before they reach authentication logic. This prevents a smuggled request from being interpreted as multiple messages. The following example shows a correct Buffalo setup using Basic Auth with strict header handling:

defmodule MyAppWeb.Router do
  use MyAppWeb, :router

  pipeline :browser do
    plug :accepts, ["html"]
    plug :fetch_session
    plug :fetch_live_flash
    plug :protect_from_forgery
    plug :put_secure_browser_headers
  end

  pipeline :api do
    plug :accepts, ["json"]
    plug MyAppWeb.BasicAuthPlug
  end

  scope "/api", MyAppWeb do
    pipe_through :api

    get "/secure", SecureController, :index
  end
end

defmodule MyAppWeb.BasicAuthPlug do
  import Plug.Conn

  def init(opts), do: opts

  def call(conn, _opts) do
    case get_req_header(conn, "authorization") do
      ["Basic " <> encoded] -
        case Base.decode64(encoded) do
          "user:pass" -> conn
          _ -> send_resp(conn, 401, "Unauthorized")
        end
      _ -> send_resp(conn, 401, "Missing Authorization")
    end
  end
end

In this setup, the plug validates the Authorization header once and rejects malformed or duplicated headers early. Combine this with input validation checks emphasized by middleBrick’s Authentication and Input Validation scans to ensure header values are properly sanitized and that no unsafe consumption of request bodies occurs. For production, prefer token-based authentication where possible, but if Basic Auth is required, enforce strict header parsing and avoid custom parsing logic that could diverge from Buffalo’s built-in behavior.