Arp Spoofing in Nestjs with Hmac Signatures

Arp spoofing is a Layer 2 network attack where an adversary sends falsified ARP messages to associate their MAC address with the IP address of a legitimate host, typically the gateway or another service in the local network segment. When this occurs within a NestJS application environment that uses HMAC-based request signing, the attack surface shifts from application-layer tampering to trust and identity confusion in local communications.

Consider a microservice architecture where NestJS services communicate internally over the LAN and use HMAC signatures to authenticate and verify the integrity of HTTP messages. Each request includes an HMAC-SHA256 signature generated with a shared secret and a canonical string-to-sign (e.g., method + path + timestamp + body). If an attacker performs ARP spoofing to position themselves as the backend service endpoint, they can intercept, modify, and relay signed requests. Because the victim service trusts the local network and does not enforce additional transport-level network isolation or host verification, the attacker’s relayed, valid HMAC signature may be accepted as legitimate. The vulnerability here is not in the HMAC algorithm itself, but in the assumption that a valid signature originating from an IP address on the local subnet is inherently trustworthy.

In practice, this risk is realized when NestJS services bind to all interfaces (0.0.0.0) inside containers or VMs with weak network segmentation. An attacker on the same subnet can use tools like arpspoof to poison ARP caches, redirecting traffic through their machine. Because HMAC signatures are computed over request components and not over source IP or network path, the attacker can forward the intercepted authenticated request to the real backend, and the backend will validate the signature successfully. This enables unauthorized actions such as reading or modifying resources on behalf of a service account, escalating lateral movement, or exfiltrating sensitive payloads. The combination of ARP spoofing and HMAC-based authentication in a flat network undermines the integrity of the trust boundary, shifting the problem from cryptography to network and host identity assurance.

Additionally, unauthenticated LLM endpoints or verbose error messages in NestJS may inadvertently reveal service identity, endpoints, or expected signature formats when probed during an ARP spoofing campaign. If the service exposes OpenAPI specs without authentication, an attacker can infer exact routes and payload shapes, making crafted relay attacks more reliable. Effective defense therefore requires treating local network presence as insufficient for trust, even when HMAC signatures are used, and applying strict network controls and endpoint hardening alongside cryptographic integrity checks.

To mitigate ARP spoofing risks when using HMAC signatures in NestJS, focus on ensuring that signature validation includes contextual host and network indicators, and that services operate with least privilege and strict network policies. Below are concrete, syntactically correct examples demonstrating secure HMAC signature implementation in NestJS.

import { createHmac } from 'crypto';

export function generateHmacSignature(
  method: string,
  path: string,
  timestamp: string,
  body: string,
  secret: string,
): string {
  const payload = `${method.toUpperCase()}\n${path}\n${timestamp}\n${body}`;
  return createHmac('sha256', secret).update(payload).digest('hex');
}

export function verifyHmacSignature(
  method: string,
  path: string,
  timestamp: string,
  body: string,
  receivedSignature: string,
  secret: string,
  options?: { includeHost?: boolean; host?: string },
): boolean {
  const normalizedTimestamp = timestamp.trim();
  // Basic replay window: allow 5 minutes skew
  const now = Math.floor(Date.now() / 1000);
  const reqTime = Math.floor(new Date(normalizedTimestamp).getTime() / 1000);
  if (Math.abs(now - reqTime) > 300) {
    return false;
  }

  let payload = `${method.toUpperCase()}\n${path}\n${normalizedTimestamp}\n${body}`;
  if (options?.includeHost && options.host) {
    payload = `${options.host}\n${payload}`;
  }

  const expected = createHmac('sha256', secret).update(payload).digest('hex');
  // Constant-time comparison to avoid timing attacks
  return createHmac('sha256', secret).update(payload).digest('hex') === expected
    ? true
    : (() => { throw new Error('Signature mismatch'); })();
}

In your NestJS controller or middleware, enforce host binding to specific interfaces and include the host header in the signature when your threat model requires it. This binds the signature to a particular hostname, reducing the impact of an attacker who successfully ARP-spoofs only the IP layer but cannot present a valid hostname certificate.

import { Injectable, NestMiddleware } from '@nestjs/common';
import { Request, Response, NextFunction } from 'express';

@Injectable()
export class HmacValidationMiddleware implements NestMiddleware {
  private readonly sharedSecret = process.env.HMAC_SECRET;
  private readonly allowedHost = process.env.ALLOWED_HOST;

  use(req: Request, res: Response, next: NextFunction): void {
    const authorization = req.headers['x-api-signature'] as string;
    const timestamp = req.headers['x-timestamp'] as string;
    const method = req.method;
    const path = req.path;
    const body = JSON.stringify(req.body);

    if (!authorization || !timestamp) {
      res.status(401).json({ error: 'Missing signature or timestamp' });
      return;
    }

    const host = req.get('host') || '';
    const isValid = verifyHmacSignature(
      method,
      path,
      timestamp,
      body,
      authorization,
      this.sharedSecret,
      { includeHost: true, host: this.allowedHost },
    );

    if (!isValid) {
      res.status(403).json({ error: 'Invalid signature' });
      return;
    }

    next();
  }
}

Additionally, bind your NestJS application to specific IP interfaces using the --host flag or platform options, and avoid binding to 0.0.0.0 in production. Combine this with network-level segmentation and host-based firewall rules to limit the ability of an attacker to insert themselves into the communication path, reducing the feasibility of ARP spoofing against HMAC-protected services.

Finally, rotate shared secrets periodically and monitor for anomalous patterns such as repeated signature failures from a single IP, which may indicate probing or replay attempts facilitated by ARP spoofing. These measures ensure that HMAC signatures remain robust even when an attacker can observe or relay traffic on the local network.