Arp Spoofing in Sails with Hmac Signatures

Arp Spoofing is a Layer 2 attack where an attacker sends falsified ARP messages to associate their MAC address with the IP address of a legitimate host, typically the default gateway. In a Sails.js application that uses Hmac Signatures to authenticate requests, Arp Spoofing can undermine the integrity of the signature verification by enabling a man-in-the-middle (MITM) position. When an attacker successfully spoofs ARP entries, they can intercept, modify, or replay HTTP requests between clients and the Sails backend. If the client computes Hmac Signatures using a shared secret and includes the signature in headers, the attacker who sits in the path can observe valid signatures for known or predictable payloads. This observation can enable replay attacks or selective modification of non-encrypted parameters, especially if the application does not enforce strict per-request nonce or timestamp validation. Because Hmac Signatures rely on the assumption that the communication channel is not actively manipulated, a successful ARP spoofing attack breaks that assumption by allowing the adversary to see and reuse authentic signatures. Additionally, if the Sails app serves both web and API clients over the same endpoints and does not segregate trust boundaries, an attacker can use ARP spoofing to pivot from a compromised client context to backend service calls that rely on Hmac Signatures for authorization, effectively bypassing intended access controls without needing to crack the secret itself.

To reduce exposure to ARP spoofing when using Hmac Signatures in Sails.js, implement strong per-request protections and ensure server-side validation is strict. Use a short-lived nonce combined with a timestamp, and include them in the signed payload so that each signature is bound to a specific request context. This prevents replay even if an attacker intercepts a valid signature. Enforce HTTPS across all endpoints to prevent passive sniffing that aids ARP spoofing, and consider mutual TLS for high-assurance scenarios. The following examples show a Sails controller that creates and verifies Hmac Signatures with nonce and timestamp protections.

// api/controllers/SignatureController.js
const crypto = require('crypto');
const NONCE_LENGTH = 16;
const TIMESTAMP_TOLERANCE_MS = 30000; // 30 seconds

function generateNonce() {
  return crypto.randomBytes(NONCE_LENGTH).toString('hex');
}

function buildStringForSignature({ method, url, timestamp, nonce, body }) {
  return `${method.toUpperCase()}\n${url}\n${timestamp}\n${nonce}\n${JSON.stringify(body)}`;
}

function computeHmac(secret, stringToSign) {
  return crypto.createHmac('sha256', secret).update(stringToSign).digest('hex');
}

module.exports = {
  sign: async function (req, res) {
    const secret = sails.config.custom.hmacSecret; // stored securely, e.g., env var
    const method = req.method;
    const url = req.originalUrl;
    const timestamp = Date.now().toString();
    const nonce = generateNonce();
    const body = req.body || {};

    const stringToSign = buildStringForSignature({ method, url, timestamp, nonce, body });
    const signature = computeHmac(secret, stringToSign);

    res.json({
      method,
      url,
      timestamp,
      nonce,
      signature,
      body
    });
  },

  verify: async function (req, res, next) {
    const secret = sails.config.custom.hmacSecret;
    const { timestamp, nonce, signature, ...body } = req.body;

    if (!timestamp || !nonce || !signature) {
      return res.status(400).json({ error: 'Missing timestamp, nonce, or signature' });
    }

    const requestTime = parseInt(timestamp, 10);
    if (isNaN(requestTime) || Math.abs(Date.now() - requestTime) > TIMESTAMP_TOLERANCE_MS) {
      return res.status(400).json({ error: 'Timestamp outside tolerance' });
    }

    const url = req.originalUrl;
    const method = req.method;
    const stringToVerify = buildStringForSignature({ method, url, timestamp, nonce, body });
    const expected = computeHmac(secret, stringToVerify);

    if (!crypto.timingSafeEqual(Buffer.from(signature), Buffer.from(expected))) {
      return res.status(401).json({ error: 'Invalid signature' });
    }

    return next();
  }
};

In your routes configuration (config/routes.js), ensure sensitive endpoints use the verify middleware:

// config/routes.js
module.exports.routes = {
  'POST /api/resource': {
    controller: 'SignatureController.verify',
    action: 'create' // or a dedicated handler
  },
  'POST /api/sign': {
    controller: 'SignatureController.sign',
    action: 'sign'
  }
};

These patterns ensure that even if an attacker can observe and replay requests via ARP spoofing, the reused nonce and stale timestamp will cause verification to fail. Keep your Hmac secret in environment variables, rotate it periodically, and audit logs for anomalies to detect possible probing or replay attempts.