Arp Spoofing in Hanami with Hmac Signatures

Arp spoofing is a Layer 2 attack where an adversary sends falsified ARP messages to associate their MAC address with the IP of a legitimate host, such as a gateway or another service in the network path. In a Hanami application that relies on Hmac Signatures for request authentication, arp spoofing can facilitate session hijacking and tampering by allowing an attacker to intercept or modify in-flight requests before they reach the intended service.

When a client computes an Hmac signature over request components (e.g., HTTP method, path, selected headers, and a timestamp or nonce) using a shared secret, the signature is meant to guarantee integrity and origin authenticity. However, if an attacker conducts arp spoofing and sits between the client and the server, they can observe the exact request data and the computed Hmac. If the implementation does not enforce strict transport integrity and does not include protections like request uniqueness and replay prevention, the attacker can replay the intercepted request with the valid Hmac to the server, and the server may accept it as legitimate.

In Hanami, which encourages explicit, domain-driven request handling, this risk is pronounced when Hmac Signatures are applied to sensitive operations (such as changing permissions or performing financial actions) without additional context-bound protections. An attacker who has poisoned the ARP cache can also alter non-Hmac parts of the request (e.g., swapping a user ID in the path or body) while keeping the signature valid if the server only verifies the signature over a limited subset of parameters. This combination exposes an authorization bypass or privilege escalation path where the attacker can act as a different user or escalate their own privileges by leveraging a valid but intercepted and mutated request.

Moreover, if the server’s Hmac verification does not validate a strict timestamp or nonce window, intercepted requests can be replayed within the validity period. In environments where requests traverse internal networks that could be subject to arp spoofing, this expands the attack surface beyond the edge to internal services that trust intra-network traffic. Therefore, relying solely on Hmac Signatures without binding them to transport protections and strict replay controls can allow arp spoofing to turn a signed request into a vehicle for unauthorized operations.

To mitigate arp spoofing risks when using Hmac Signatures in Hanami, you must ensure that signatures cover all mutable and attacker-influenced parts of the request, enforce strict replay and freshness controls, and avoid trusting network-level assumptions. Below are concrete patterns and code examples for a more robust implementation.

• Include a nonce and timestamp in the Hmac scope and enforce a tight validity window on the server. This prevents replay even if an attacker observes a valid signature via arp spoofing.
• Bind the signature to the request body and exact set of headers to prevent tampering with critical parameters that the server might otherwise trust implicitly.
• Use constant-time comparison for Hmac verification to avoid timing side channels that could aid an attacker in refining forged requests.

Example request signing on the client (Ruby) for Hanami, including timestamp and nonce:

require "openssl"
require "base64"
require "json"
require "time"

def build_auth_header(method, path, body, secret)
  timestamp = Time.now.utc.iso8601
  nonce = SecureRandom.uuid
  payload = [method.upcase, path, timestamp, nonce, body].join("|")
  signature = OpenSSL::HMAC.hexdigest("SHA256", secret, payload)
  {"Authorization" => "Hmac timestamp=\"#{timestamp}\", nonce=\"#{nonce}\", signature=\"#{signature}\""}
end

# Usage
secret = ENV["HMAC_SECRET"]
headers = build_auth_header("POST", "/api/v1/transfers", '{"amount":100,"to":"acct_2"}', secret)
# Include headers in your Hanami HTTP client request

Server-side verification in Hanami, with replay and freshness checks:

class Auth::HmacVerifier
  MAX_AGE = 60 # seconds

  def initialize(request)
    @request = request
  end

  def valid?
    return false unless header
    parts = parse_header
    return false unless parts[:timestamp] && parts[:nonce] && parts[:signature]
    return false if (Time.now.utc - Time.iso8601(parts[:timestamp])).to_i > MAX_AGE
    return false if redis_set.include?(parts[:nonce]) # replay protection
    payload = [request.request_method, request.path, parts[:timestamp], parts[:nonce], request.raw_body].join("|")
    expected = OpenSSL::HMAC.hexdigest("SHA256", ENV["HMAC_SECRET"], payload)
    secure_compare(expected, parts[:signature])
  ensure
    redis_set.add(parts[:nonce]) if parts[:nonce]
  end

  private

  def parse_header
    header = @request.headers["Authorization"]
    return {} unless header && header.start_with?("Hmac")
    header.slice("timestamp", "nonce", "signature").transform_values { |v| v.to_s.gsub(/^\"|\"$/, "") }
  end

  def secure_compare(a, b)
    ActiveSupport::SecurityUtils.secure_compare(a, b)
  end

  def redis_set
    # Use a distributed cache with TTL slightly larger than MAX_AGE
    Redis.current
  end
end

In addition to these code-level fixes, consider layering protections: enforce TLS to reduce the effectiveness of arp spoofing, use mutual TLS where feasible, and avoid relying on network position for trust. For continuous assurance, integrate middleBrick to scan your API endpoints and surface misconfigurations related to authentication and input validation that could amplify the impact of network-layer attacks. Whether you use the middleBrick CLI to scan from terminal with middlebrick scan or incorporate the GitHub Action to add API security checks to your CI/CD pipeline, you can detect issues before they reach production.