Third-party middleware (cors, helmet) in NestJS

Introduction

Third-party middleware like cors and helmet help protect your NestJS app and make it work well with other websites.

When you want to allow your API to be accessed from other websites safely (use CORS).

When you want to add security headers to protect your app from common web attacks (use Helmet).

When you build a public API that many clients will use from different domains.

When you want to quickly add security best practices without writing complex code.

When you want to control who can access your backend resources.

Syntax

NestJS

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

async function bootstrap() {
  const app = await NestFactory.create(AppModule);

  app.use(cors());
  app.use(helmet());

  await app.listen(3000);
}

Use app.use() to add middleware in NestJS.

Both cors() and helmet() are functions that return middleware.

Examples

This enables default CORS settings, allowing all origins.

NestJS

app.use(cors());

This restricts access to only https://example.com.

NestJS

app.use(cors({ origin: 'https://example.com' }));

This adds many security headers automatically.

NestJS

app.use(helmet());

This sets a strict content security policy to allow resources only from your own site.

NestJS

app.use(helmet.contentSecurityPolicy({ directives: { defaultSrc: ["'self'"] } }));

Sample Program

This NestJS app uses cors middleware to allow cross-origin requests and helmet middleware to add security headers. When you run it, the server listens on port 3000.

NestJS

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

async function bootstrap() {
  const app = await NestFactory.create(AppModule);

  // Enable CORS for all origins
  app.use(cors());

  // Add security headers
  app.use(helmet());

  await app.listen(3000);
  console.log('Server running on http://localhost:3000');
}

bootstrap();

Important Notes

Always configure CORS carefully to avoid exposing your API to unwanted sites.

Helmet sets many headers by default, but you can customize it for your needs.

Use browser DevTools Network tab to check CORS headers and security headers in responses.

Summary

CORS middleware controls which websites can access your API.

Helmet middleware adds security headers to protect your app.

Use app.use() in NestJS to add these middlewares easily.

Practice

1. What is the main purpose of using cors middleware in a NestJS application?

easy

A. To add security headers to HTTP responses

B. To control which websites can access your API

C. To log all incoming requests

D. To compress response data for faster delivery

Solution

1. Step 1: Understand the role of CORS

   CORS (Cross-Origin Resource Sharing) middleware controls which external websites or domains can access your API resources.

2. Step 2: Differentiate from other middleware

   Helmet adds security headers, logging middleware logs requests, and compression middleware compresses data. Only CORS controls access from other origins.

3. Final Answer:

   To control which websites can access your API -> Option B

4. Quick Check:

   CORS controls access = B [OK]

Hint: Remember CORS controls cross-site access permissions [OK]

Common Mistakes:

• Confusing CORS with Helmet's security headers
• Thinking CORS logs requests
• Assuming CORS compresses data

2. Which of the following is the correct way to apply Helmet middleware in a NestJS app?

easy

A. app.apply(helmet)

B. app.helmet()

C. app.use(helmet())

D. app.useHelmet()

Solution

1. Step 1: Recall NestJS middleware usage

   In NestJS, third-party middleware like Helmet is applied using app.use() with the middleware function called.

2. Step 2: Check syntax correctness

   The correct syntax is app.use(helmet()). Other options are not valid NestJS methods or miss calling the function.

3. Final Answer:

   app.use(helmet()) -> Option C

4. Quick Check:

   Use app.use() with middleware function call = A [OK]

Hint: Use app.use() with middleware function call [OK]

Common Mistakes:

• Forgetting to call helmet as a function
• Using non-existent app methods like app.helmet()
• Passing helmet without parentheses

3. Given this NestJS setup code, what will be the effect of the middleware?

import * as helmet from 'helmet';
import * as cors from 'cors';

async function bootstrap() {
  const app = await NestFactory.create(AppModule);
  app.use(cors({ origin: 'https://example.com' }));
  app.use(helmet());
  await app.listen(3000);
}

medium

A. Only requests from https://example.com are allowed, and security headers are added

B. All requests are allowed, but no security headers are added

C. Requests from any origin are blocked, and no headers are added

D. Requests from https://example.com are blocked, but security headers are added

Solution

1. Step 1: Analyze CORS middleware configuration

   The CORS middleware is configured to allow only requests from 'https://example.com'. Requests from other origins will be blocked.

2. Step 2: Analyze Helmet middleware effect

   Helmet adds security headers to all responses to improve security.

3. Final Answer:

   Only requests from https://example.com are allowed, and security headers are added -> Option A

4. Quick Check:

   CORS origin restrict + Helmet headers = C [OK]

Hint: Check CORS origin and Helmet adds headers [OK]

Common Mistakes:

• Assuming all origins are allowed
• Thinking Helmet blocks requests
• Confusing CORS blocking with allowing

4. Identify the error in this NestJS middleware setup:

import * as cors from 'cors';

async function bootstrap() {
  const app = await NestFactory.create(AppModule);
  app.use(cors);
  await app.listen(3000);
}

medium

A. Missing await before app.use(cors)

B. cors should be imported from '@nestjs/cors' instead

C. app.use() cannot be used for middleware

D. Missing parentheses to call cors function

Solution

1. Step 1: Check how cors is used

   The code passes cors directly without calling it as a function. Middleware functions must be called to return the actual middleware.

2. Step 2: Confirm correct usage

   The correct usage is app.use(cors()). Other options are incorrect because NestJS supports app.use(), and no await is needed for middleware.

3. Final Answer:

   Missing parentheses to call cors function -> Option D

4. Quick Check:

   Middleware must be called as function = A [OK]

Hint: Always call middleware functions with () in app.use() [OK]

Common Mistakes:

• Passing middleware reference without calling it
• Confusing import sources for cors
• Trying to await app.use()

5. You want to secure your NestJS API by allowing only requests from https://myapp.com and add security headers. Which code snippet correctly applies both cors and helmet middleware with these requirements?

hard

A. app.use(cors({ origin: 'https://myapp.com' })); app.use(helmet());

B. app.use(helmet()); app.use(cors());

C. app.use(cors()); app.use(helmet({ origin: 'https://myapp.com' }));

D. app.use(cors({ origin: '*' })); app.use(helmet());

Solution

1. Step 1: Configure CORS with specific origin

   To allow only 'https://myapp.com', pass { origin: 'https://myapp.com' } to cors middleware.

2. Step 2: Apply Helmet middleware correctly

   Helmet is applied with helmet() to add security headers. It does not take origin options.

3. Step 3: Verify order and correctness

   Applying CORS first with correct origin, then Helmet, matches the requirements. Other options either miss origin restriction or misuse Helmet options.

4. Final Answer:

   app.use(cors({ origin: 'https://myapp.com' })); app.use(helmet()); -> Option A

5. Quick Check:

   CORS origin set + Helmet default = D [OK]

Hint: Set origin in cors, call helmet() without options [OK]

Common Mistakes:

• Passing origin option to helmet instead of cors
• Allowing all origins with '*'
• Reversing middleware order causing unexpected behavior