Replay Attack in Grape with Basic Auth

A replay attack occurs when an attacker intercepts a valid request and retransmits it to reproduce the original effect. In Grape, using HTTP Basic Authentication over non-HTTPS transport creates a high-risk scenario because the credentials are static per request and can be captured and reused. Basic Auth encodes the username and password with Base64, which is trivial to decode; without TLS, the encoded string travels in plaintext and is easily observed on the network. An attacker who observes a single authenticated request can replay the same method, path, and headers to the server, and Grape may treat the replay as legitimate because the credentials remain valid.

Even when TLS is used, replay remains possible if nonces, timestamps, or request-specific counters are not employed. Grape, being a lightweight API framework, does not enforce replay protection by default; it relies on transport security and application-level safeguards. Without additional mechanisms such as idempotency keys, one-time tokens, or strict timestamp windows, an attacker can reuse captured requests to perform unauthorized operations like transferring funds or modifying user settings. This risk is amplified in unauthenticated scanning scenarios where an endpoint is probed without credentials; a scanner can still observe and replay authentication challenges if defenses are not explicit.

Moreover, if the same Basic Auth credentials are shared across multiple clients or services, the attack surface increases because capturing credentials from one compromised channel enables access to all systems using those credentials. The combination of static credentials, predictable request patterns, and missing replay controls means that an intercepted request can be reused at a later time, potentially outside the intended time window. middleBrick scans identify such weaknesses by testing the unauthenticated attack surface and flagging endpoints that accept repeated identical requests without replay mitigation, providing prioritized findings with severity ratings and remediation guidance to help developers address the issue.

To mitigate replay attacks in Grape while using Basic Auth, you should combine transport security with request-level protections. Always enforce HTTPS to prevent eavesdropping on credentials and to protect the integrity of each request. In addition, introduce nonces or short-lived timestamps combined with server-side validation to ensure that each request is unique and time-bound. Below are concrete code examples that demonstrate how to implement secure Basic Auth in Grape with replay-resistant measures.

# Gemfile
gem 'grape'
gem 'rack' # for request utilities

# app/api/protected_api.rb
require 'grape'
require 'base64'
require 'openssl'
require 'securerandom'

class ProtectedAPI < Grape::API
  format :json

  # Middleware to enforce HTTPS in production (conceptual, use proper SSL termination in practice)
  before do
    # Reject HTTP in environments where TLS is required
    # if !request.secure? && ENV['RACK_ENV'] == 'production'
    #   error!('HTTPS required', 403)
    # end
  end

  helpers do
    def authenticate_with_basic
      auth_header = request.env['HTTP_AUTHORIZATION']
      unless auth_header&.start_with?('Basic ')
        throw(:error, { message: 'Missing or invalid Authorization header', status: 401 })
      end
      decoded = Base64.strict_decode64(auth_header.split(' ').last)
      username, password = decoded.split(':', 2)
      # Replace with secure credential validation, e.g., hashed comparison
      unless valid_credentials?(username, password)
        throw(:error, { message: 'Invalid credentials', status: 401 })
      end
      { username: username }
    rescue ArgumentError
      throw(:error, { message: 'Invalid Basic Auth encoding', status: 400 })
    end

    def valid_credentials?(username, password)
      # Use constant-time comparison and a secure store
      expected_password = ENV['API_USER_PASSWORD'] # In practice, use a secure vault or hashed lookup
      password == expected_password
    end

    def replay_protection
      # Simple nonce/timestamp approach: require X-Request-Nonce and X-Request-Timestamp
      nonce = request.env['HTTP_X_REQUEST_NONCE']
      timestamp = request.env['HTTP_X_REQUEST_TIMESTAMP']
      unless nonce && timestamp
        throw(:error, { message: 'Missing replay protection headers', status: 400 })
      end
      # Reject if timestamp is too old (e.g., more than 2 minutes)
      request_time = Time.at(timestamp.to_f)
      if request_time < (Time.now - 120)
        throw(:error, { message: 'Request expired', status: 400 })
      end
      # Store nonce server-side (e.g., in Redis with TTL) to detect reuse
      # pseudo: if seen_nonce(nonce); throw(:error, { message: 'Replay detected', status: 403 }); end
    end
  end

  before { authenticate_with_basic }
  before { replay_protection }

  resource :secure do
    desc 'A protected endpoint with Basic Auth and replay protection'
    get do
      { message: 'Access granted', user: current_user }
    end
  end
end

In production, store nonces or one-time tokens in a fast, transient store such as Redis with a short TTL to detect reuse across requests. Combine this with framework-level or gateway-level rate limiting to reduce the likelihood of successful replays. middleBrick’s scans can validate that endpoints include these protections and highlight endpoints that accept repeated requests without nonce or timestamp checks, helping you prioritize fixes.