Brute Force Attack in Buffalo with Bearer Tokens

A brute force attack against a Buffalo application that relies on Bearer Tokens can be especially effective when token issuance or validation logic does not adequately limit authentication attempts. In this scenario, an attacker systematically guesses or enumerates valid tokens by making repeated requests to token verification endpoints or resource endpoints that accept Authorization: Bearer <token>. Because Buffalo does not enforce rate limiting or account lockout at the token-checking layer, each request returns a distinct HTTP status and timing signal that can be leveraged to infer validity without requiring user credentials.

When Bearer Tokens are issued with high entropy but still follow predictable patterns (e.g., sequential IDs embedded in the token, or insufficient randomness in the token string), an attacker can perform token enumeration. For example, an API route like /api/invoices/:id that validates a Bearer Token in the header may inadvertently disclose whether a given token is authorized for a specific resource through response differences (200 vs 403 vs 404). This unauthenticated attack surface is precisely what middleBrick tests as part of its BOLA/IDOR and Authentication checks, flagging cases where token validation does not consistently enforce authorization regardless of token validity.

The combination of Buffalo routes, Bearer Token usage, and missing rate limiting creates a vulnerability chain: an attacker can send many requests with different tokens, observe timing differences, and harvest valid tokens through trial and error. This maps to the OWASP API Top 10 category Broken Object Level Authorization (BOLA) and can also intersect with Excessive Agency when token scopes are overly permissive. middleBrick’s LLM/AI Security checks further ensure that token validation logic is not inadvertently exposed through model outputs or debug endpoints, which could aid an attacker in refining brute force attempts.

Remediation focuses on ensuring that token validation is constant-time, uniformly enforced, and coupled with strict rate limiting. In Buffalo, you should centralize token verification in a plug or assign a before action that validates the Bearer Token using a secure comparison function and returns a generic error for any invalid token, preventing enumeration via status code differences.

Example: Secure Bearer Token validation in a Buffalo controller

defmodule MyAppWeb.InvoiceController do
  use MyAppWeb, :controller

  plug MyAppWeb.Plug.EnsureBearerToken when action in [:show, :update, :delete]

  def show(conn, %{"id" => id}) do
    # The token has already been validated and bound to conn.assigns.current_token
    invoice = Invoices.get_invoice!(id, conn.assigns.current_token)
    render(conn, "show.json", invoice: invoice)
  end
end

Example: Constant-time token verification plug

defmodule MyAppWeb.Plug.EnsureBearerToken do
  import Plug.Conn
  alias MyApp.Accounts

  def init(opts), do: opts

  def call(conn, _opts) do
    case get_bearer_token(conn) do
      nil -> send_unauthorized(conn)
      token ->
        # Use a constant-time comparison to avoid timing leaks
        if Accounts.valid_bearer_token?(token) do
          assign(conn, :current_token, token)
        else
          send_unauthorized(conn)
        end
    end
  end

  defp get_bearer_token(conn) do
    case get_req_header(conn, "authorization") do
      ["Bearer " <> token] -> token
      _ -> nil
    end
  end

  defp send_unauthorized(conn) do
    conn
    |> put_status(:unauthorized)
    |> json(%{error: "unauthorized"})
    |> halt()
  end
end

Example: Enforcing rate limits on token validation endpoints

defmodule MyAppWeb.Plugs.RateLimitTokens do
  use Plug.Router
  import Plug.Conn

  # Simple in-memory rate limit for demo; use Redis or other shared store in production
  @limit 30               # requests
  @period 60              # per 60 seconds

  def init(opts), do: opts

  def call(conn, _) do
    key = {"rate_limit", conn.ip, token_key(conn)}
    current = :ets.lookup_element(:ratetable, key, 2) rescue 0
    if current < @limit do
      :ets.insert(:ratetable, {key, current + 1, System.system_time(:second)})
      conn
    else
      conn
      |> put_status(429)
      |> json(%{error: "too many requests"})
      |> halt()
    end
  end

  defp token_key(conn) do
    case get_req_header(conn, "authorization") do
      ["Bearer " <> token] -> {:token, token}
      _ -> {:anonymous, conn.ip}
    end
  end
end

Operational and architectural guidance

• Always return the same generic error payload and HTTP status for invalid tokens (e.g., 401 Unauthorized) to prevent attackers from distinguishing valid from invalid tokens.
• Use cryptographically secure random tokens with sufficient entropy (e.g., 256-bit) and avoid embedding user or sequential identifiers inside the token string.
• Apply rate limiting at the API gateway or connection level, not only within application code, to protect against high-volume brute force attempts.
• Validate token scopes and permissions server-side for every request, ensuring that even if a token is brute-forced, its impact is limited by principle of least privilege.

These changes align with findings from security scans that map to compliance frameworks such as OWASP API Top 10 and help reduce the attack surface for token-based brute force without introducing breaking changes to existing Buffalo routes.