Arp Spoofing in Sinatra with Basic Auth

Arp spoofing is a link-layer attack where an attacker sends falsified Address Resolution Protocol replies to associate their MAC address with the IP address of another host, typically the default gateway. In a Sinatra application that uses HTTP Basic Auth, this attack becomes especially consequential because credentials are transmitted in an easily recoverable form.

When a Sinatra app relies solely on Basic Auth over HTTP, the username and password are encoded with Base64 and sent in the Authorization header of each request. An attacker who successfully performs ARP spoofing on the local network can intercept this traffic, decode the Base64 string, and recover the plaintext credentials without needing to break encryption. Because Basic Auth does not encrypt the credentials, the attack effectively exposes authentication details in transit.

The combination of Sinatra, Basic Auth, and an unsegmented or trusted local network increases the attack surface. If the Sinatra server is hosted in an environment where other devices are on the same broadcast domain (for example, shared office or cloud host network interfaces), the opportunity for ARP manipulation rises. The attacker does not need to compromise the server itself; they only need to position themselves in the network path. Once the attacker’s machine responds to ARP requests for the gateway or the Sinatra server, traffic routes through the attacker, enabling passive sniffing and potential modification of requests and responses.

Because middleBrick scans the unauthenticated attack surface, it can detect indicators such as the presence of Basic Auth without Transport Layer Security and flag the risk of credential exposure in transit. Although the scanner does not perform active ARP spoofing, it highlights the absence of encryption as a high-severity finding. A scanner run may surface related issues like missing HTTP Strict Transport Security (HSTS) and lack of secure flag usage on cookies, which compound the risk in an environment vulnerable to link-layer attacks.

It is important to note that ARP spoofing occurs at the network layer and is not something that Sinatra or the application code alone can prevent. The mitigation must involve network controls, transport security, and secure authentication mechanisms. middleBrick’s findings in this context direct attention toward enforcing encryption and hardening the authentication flow rather than attempting to detect ARP frames at the application level.

To protect credentials in a Sinatra application, the primary remediation is to enforce HTTPS so that all communication, including the Authorization header, is encrypted in transit. Below are concrete code examples that demonstrate a secure approach using HTTP Basic Auth over HTTPS, along with ancillary protections such as secure cookies and HSTS.

Enforcing HTTPS in Sinatra

Ensure your Sinatra app is deployed behind a TLS-terminating proxy or web server (e.g., NGINX, HAProxy, or a cloud load balancer). Configure your app to require secure cookies and to redirect HTTP to HTTPS.

# config.ru or within the Sinatra app file
require 'sinatra'
require 'rack' 

# Redirect all HTTP traffic to HTTPS
configure do
  set :bind, '0.0.0.0'
  set :port, 80
end

before do
  # Enforce HTTPS by redirecting
  if request.env['HTTP_X_FORWARDED_PROTO'] == 'http' || (request.secure? == false && request.env['HTTP_HOST'] !~ /^localhost/) 
    redirect "https://#{request.env['HTTP_HOST']}#{request.env['REQUEST_URI']}", 301
  end
end

Secure Basic Auth implementation over HTTPS

When using Basic Auth, validate credentials over HTTPS and avoid logging or exposing the header. Use secure, HTTP-only cookies for session management when possible, and avoid sending credentials on every request once authenticated.

# app.rb
require 'sinatra'
require 'base64'
require 'securerandom'

# In production, store credentials hashed in a secure vault or environment variables
VALID_USERNAME = ENV.fetch('API_USER', 'admin')
VALID_PASSWORD_HASH = ENV.fetch('API_PASS_HASH') { BCrypt::Password.create(ENV.fetch('PLAIN_PASSWORD', 'secret')) }

helpers do
  def authenticate!
    # Require Authorization header
    unless request.env['HTTP_AUTHORIZATION']&.start_with?('Basic ')
      halt 401, { 'WWW-Authenticate' => 'Basic realm="API"' }.to_json
    end

    encoded = request.env['HTTP_AUTHORIZATION'].split(' ').last
    decoded = Base64.strict_decode64(encoded)
    username, password = decoded.split(':', 2)

    # Constant-time comparison to reduce timing attack surface
    unless username == VALID_USERNAME && BCrypt::Password.new(VALID_PASSWORD_HASH) == password
      halt 401, { 'WWW-Authenticate' => 'Basic realm="API"' }.to_json
    end
  end
end

# Example protected endpoint
before '/api/*' do
  authenticate!
  # Ensure responses are served over HTTPS
  request.secure? or halt 400, { error: 'HTTPS required' }.to_json
end

get '/api/health' do
  { status: 'ok' }.to_json
end

Complementary protections

• Set the Secure and HttpOnly flags on any cookies used by the application.
• Add HSTS headers to instruct browsers to always use HTTPS.
• Use environment variables or a secrets manager for credentials and hashes; do not hardcode secrets.
• Consider moving from Basic Auth to token-based authentication (e.g., OAuth 2.0 or JWT) for improved security and revocation capabilities.

These code examples align with the remediation guidance provided in middleBrick findings, emphasizing encryption, secure credential handling, and proper use of authentication headers. By combining these application-level changes with network-level protections, the risk associated with ARP spoofing and credential interception is substantially reduced.