Arp Spoofing in Nestjs with Bearer Tokens

Arp spoofing is a Layer 2 attack where an attacker sends falsified Address Resolution Protocol messages to associate their MAC address with the IP address of a legitimate host, typically the default gateway. In a NestJS application that relies on Bearer Tokens for authentication, arp spoofing does not directly compromise the token itself, but it creates conditions that enable token interception and session hijacking on the local network segment.

When a NestJS service validates a Bearer Token on each request, the assumption is often that the transport between client and server is confidential and tamper-proof. If an attacker performs arp spoofing between the client and the server, they can position themselves as a man-in-the-middle on the same local network. The attacker can observe, modify, or replay HTTP traffic. While HTTPS prevents passive decryption, a compromised or intentionally misconfigured local environment (such as development setups or services without strict certificate validation) may allow the attacker to terminate or alter the TLS session. In such scenarios, Bearer Tokens sent in the Authorization header can be captured, logged, or replayed by the attacker.

Additionally, arp spoofing can facilitate further attacks that impact the integrity of the NestJS application itself. For example, an attacker might redirect traffic to a malicious server that presents a valid but fraudulent API endpoint. If the client does not strictly validate the server’s certificate or hostname, the Bearer Token can be sent to the attacker. This is especially risky in microservice architectures where services communicate internally over HTTP and depend on network-level trust rather than strict per-request token validation. The combination of arp spoofing and Bearer Token usage therefore exposes a critical trust boundary: the network layer is assumed secure, but a local attacker can bypass this assumption without needing to compromise the token generation mechanism.

Real-world attack patterns such as session hijacking commonly exploit this vector. While this does not involve a specific CVE tied to NestJS itself, the risk aligns with OWASP API Security Top 10 categories such as Broken Object Level Authorization and Insufficient Transport Layer Protection. MiddleBrick scans detect scenarios where unauthenticated API endpoints are exposed and where security configurations may allow interception, providing findings mapped to these frameworks along with remediation guidance.

To mitigate arp spoofing risks in a NestJS application using Bearer Tokens, focus on ensuring that every request validates the token securely, that communication is protected, and that the application does not implicitly trust the network. Below are concrete code examples and configurations for secure Bearer Token handling.

1. Always validate Bearer Tokens on each request

Use an AuthGuard to verify the token before allowing access to controllers. This ensures that even if traffic is intercepted, the token is checked against your authorization logic on every call.

import { Injectable, CanActivate, ExecutionContext } from '@nestjs/common';
import { Observable } from 'rxjs';

@Injectable()
export class AuthGuard implements NestCanActivate {
  canActivate(context: ExecutionContext): boolean | Promise<boolean> | Observable<boolean> {
    const request = context.switchToHttp().getRequest();
    const authHeader = request.headers['authorization'];
    const token = authHeader && authHeader.split(' ')[1];

    if (!token) {
      throw new UnauthorizedException('Missing Bearer Token');
    }

    // Replace with your actual token verification logic, e.g., JWT verification
    const isValid = this.verifyToken(token);
    if (!isValid) {
      throw new UnauthorizedException('Invalid Bearer Token');
    }
    return true;
  }

  private verifyToken(token: string): boolean {
    // Implement signature verification, expiration checks, and audience/issuer validation
    // Example for JWT: return jwt.verify(token, publicKey, { algorithms: ['RS256'] });
    return token === process.env.API_BEARER_TOKEN; // Simplified example
  }
}

2. Enforce HTTPS and strict TLS settings

Ensure that all API endpoints are served over HTTPS and that the NestJS server rejects insecure connections. Use environment variables for certificate paths and avoid disabling certificate verification.

import { NestFactory } from '@nestjs/core';
import { AppModule } from './app.module';
import * as https from 'https';
import * as fs from 'fs';

async function bootstrap() {
  const app = await NestFactory.create(AppModule);
  const httpsOptions = {
    key: fs.readFileSync('/path/to/private.key'),
    cert: fs.readFileSync('/path/to/certificate.crt'),
  };
  await app.listen(443, '0.0.0.0', () => {
    console.log('HTTPS server running on port 443');
  });
}
bootstrap();

3. Avoid logging or exposing Bearer Tokens

Ensure that request and response logging does not include the Authorization header. Configure your logging module to filter sensitive headers.

import { NestFactory } from '@nestjs/core';
import { AppModule } from './app.module';
import { WINSTON_MODULE_NEST_PROVIDER } from 'nest-winston';

async function bootstrap() {
  const app = await NestFactory.create(AppModule, {
    logger: [WINSTON_MODULE_NEST_PROVIDER],
  });
  app.use((req, res, next) => {
    if (req.headers['authorization']) {
      req.headers['authorization'] = '[Filtered]';
    }
    next();
  });
  await app.listen(3000);
}
bootstrap();

4. Use secure token storage and short lifetimes

Store Bearer Tokens securely on the client side and use short expiration times. Refresh tokens should be handled with additional safeguards and never transmitted over insecure channels.