Bleichenbacher Attack in Buffalo with Basic Auth

A Bleichenbacher attack is a cryptographic padding oracle attack originally described against PKCS#1 v1.5 RSA encryption. In the context of an HTTP API served by Buffalo using HTTP Basic Authentication, the term refers to a scenario where error differentiation in authentication responses leaks information that an attacker can exploit to recover credentials or session tokens without knowing the plaintext password. Buffalo applications that use Basic Auth typically decode the base64-encoded Authorization: Basic <credentials> header, validate the username and password, and return different HTTP statuses or response bodies depending on whether the username is valid, the password is wrong, or the credentials are malformed.

When such endpoints exhibit timing differences or distinct error messages—such as 401 with WWW-Authenticate for a valid user and 400 or no WWW-Authenticate for an unknown user—an attacker can treat the server as a padding oracle. By sending many crafted Basic credentials and measuring response times or inspecting error details, the attacker iteratively decrypts or deduces the protected secret. This is especially relevant when the underlying credential verification uses a block cipher in a mode susceptible to padding queries, and the API surface is unauthenticated (black-box scanning) as provided by middleBrick, which tests authentication mechanisms without credentials.

Because middleBrick scans the unauthenticated attack surface and includes an Authentication check among its 12 parallel security checks, it can surface differences in server behavior that indicate a potential oracle condition. For example, a scan might observe that certain usernames elicit a 401 with a WWW-Authenticate header while others elicit 400 Bad Request, even when the overall API security score is not immediately failing. These findings map to the Authentication control and highlight the risk when error handling and timing are not constant time.

In Buffalo, developers often use middleware or filters to perform Basic Auth. If the validation logic is not carefully constructed, subtle branches or string comparisons can introduce the very timing or error distinctions that enable a Bleichenbacher-style attack. Because the attack exploits informational leaks rather than a direct implementation flaw in crypto libraries, remediation focuses on making authentication responses uniform and side-channel resistant.

To mitigate Bleichenbacher-style risks when using HTTP Basic Authentication in Buffalo, ensure that all authentication paths execute in constant time and return identical HTTP status codes and response shapes for invalid credentials. Avoid early exits based on username existence, and normalize error responses so that an attacker cannot distinguish between a missing user and a wrong password.

Below are concrete examples for Buffalo applications. The safe pattern uses a fixed delay and a generic 401 response regardless of whether the user exists.

Safe Basic Auth Handler

use plug.Conn

# Simulated user lookup (replace with your data layer)
defmodule MyApp.User do
  def find_by_username(username) do
    # In real code, query your database or directory
    case username do
      "alice" -> %{username: "alice", password_hash: "$2b$12$..."}
      _ -> nil
    end
  end
end

# A plug that performs constant-time validation
defmodule MyApp.BasicAuthPlug do
  import Plug.Conn

  # A constant-time comparison helper to avoid timing leaks
  defp constant_time_compare(a, b) when byte_size(a) == byte_size(b) do
    do_compare(binary_to_list(a), binary_to_list(b), 0)
  end
  defp constant_time_compare(_a, _b), do: false

  defp do_compare([], [], acc), do: acc == 0
  defp do_compare([ha | ta], [hb | tb], acc) do
    do_compare(ta, tb, acc ||| (ha ^^^ hb))
  end

  def init(opts), do: opts

  def call(conn, _opts) do
    auth_header = get_req_header(conn, "authorization") |> List.first()
    user = case auth_header do
      "Basic " <> encoded ->
        # Decode and split without branching on user existence
        decoded = Base.url_decode64!(encoded, padding: false)
        [username, password] = String.split(decoded, ":", parts: 2, trim: true)
        MyApp.User.find_by_username(username)
      _ ->
        nil
    end

    # Simulate work to obscure timing differences
    :timer.sleep(100)  # constant delay in milliseconds

    # Always return the same status shape for invalid credentials
    cond do
      user && constant_time_compare(user.password_hash, hash_password(password)) ->
        # Attach user for downstream handlers
        assign(conn, :current_user, user)
      true ->
        # Identical response for any invalid input
        conn
        |> put_resp_header("www-authenticate", "Basic realm=\"api\"")
        |> send_resp(401, "{\"error\": \"unauthorized\"}")
        |> halt()
    end
  end
end

Key points in the remediation:

• Always decode the header and split credentials before checking user existence.
• Use a constant-time comparison to compare password hashes, avoiding early-exit branches.
• Return the same HTTP status code and similar response body for all invalid authentication attempts.
• Introduce a small, fixed delay to obscure timing differences that could feed an oracle.

If you rely on Buffalo’s built-in session or pipeline mechanisms, ensure that any failure path calls send_resp/2 with a uniform 401 and does not vary status codes based on validation stage. middleBrick’s Authentication checks can help identify remaining variations in response behavior across endpoints.