Arp Spoofing in Fiber with Hmac Signatures

Arp Spoofing is a Layer 2 attack where an attacker sends falsified ARP messages onto a local network to associate their MAC address with the IP address of a legitimate host, such as a gateway or another service. In a Fiber-based application using Hmac Signatures for request authentication, the attack does not directly break the cryptographic integrity of the Hmac itself, but it creates a position of control that can weaken trust in the surrounding infrastructure. When an attacker successfully spoofs ARP entries, they can intercept, modify, or redirect HTTP traffic between clients and the Fiber server. If the client is tricked into sending requests to the attacker’s machine, the attacker can observe or manipulate unencrypted portions of the request, such as headers and query parameters, before relaying them to the legitimate server. Because Hmac Signatures are typically computed over selected parts of the request—such as method, path, selected headers, and a timestamp—an attacker who can intercept and alter the request before it reaches the server can attempt to probe for weak canonicalization or timing differences in signature validation. Additionally, if the client verifies server identity only via DNS or IP and not via certificate pinning, a man-in-the-middle position established through Arp Spoofing may enable the attacker to serve responses that appear valid if the client does not enforce strict host verification. In environments where multiple services share a local network, such as microservices communicating internally, a compromised host can use Arp Spoofing to position itself as a relay, collecting authenticated requests that use Hmac Signatures and attempting to reuse them if nonces or timestamps are not strictly enforced. This combination therefore exposes operational risks around request replay and trust in network-layer identity, even when cryptographic signatures are present.

To reduce risk when using Hmac Signatures in Fiber, enforce strict request canonicalization and validate metadata that influences signature generation. Ensure that the signature is computed over a deterministic string that includes the HTTP method, path, selected headers in a consistent order, and an expiring timestamp or nonce. The server must reject requests with timestamps or nonces that are out of an allowed window and must not reuse nonce values. Below is a concrete example of computing and verifying Hmac Signatures in a Fiber application using Node.js and the built-in crypto module.

const crypto = require('crypto');

const SHARED_SECRET = process.env.HMAC_SHARED_SECRET; // store securely

function buildSignatureString(req) {
  const method = req.method.toUpperCase();
  const path = req.path; // ensure no query string unless explicitly included
  const timestamp = req.get('X-Request-Timestamp');
  const nonce = req.get('X-Nonce');
  // Include selected headers if needed, e.g., X-Client-Id
  const headersToSign = ['x-client-id']
    .map(h => h.toLowerCase() + ':' + (req.get(h) || ''))
    .join('\n');
  // Canonical representation
  return [method, path, timestamp, nonce, headersToSign].join('\n');
}

function computeHmac(body, signatureString) {
  return crypto.createHmac('sha256', SHARED_SECRET)
    .update(signatureString)
    .update(body)
    .digest('hex');
}

// Middleware to verify signature in Fiber
const { app } = require('@gofiber/app');
const crypto = require('crypto');

app.post('/api/action', (req, res) => {
  const received = req.get('X-Signature');
  if (!received) {
    return res.status(401).json({ error: 'missing_signature' });
  }
  const timestamp = req.get('X-Request-Timestamp');
  if (!timestamp || Math.abs(Date.now() / 1000 - parseInt(timestamp, 10)) > 300) {
    return res.status(400).json({ error: 'stale_timestamp' });
  }
  const nonce = req.get('X-Nonce');
  if (!nonce) {
    return res.status(400).json({ error: 'missing_nonce' });
  }
  // Ensure nonce uniqueness in your store; simplified here
  const signatureString = buildSignatureString(req);
  const expected = computeHmac(req.body, signatureString);
  if (!crypto.timingSafeEqual(Buffer.from(received), Buffer.from(expected))) {
    return res.status(403).json({ error: 'invalid_signature' });
  }
  res.json({ status: 'ok' });
});

On the client, construct the same canonical string and include the signature in a header such as X-Signature, along with X-Request-Timestamp and X-Nonce. Always use HTTPS to prevent on-path observers from altering the request in transit. To further harden against replay and Arp Spoofing–enabled interception, implement short-lived timestamps (for example, 5 minutes), enforce nonce uniqueness server-side, and validate the Host header or use certificate pinning in clients to reduce reliance on network-layer identity alone. These measures ensure that even if an attacker positions themselves via Arp Spoofing, they cannot produce valid Hmac Signatures without the shared secret and cannot reuse captured requests meaningfully.