CSRF protection in Express

Introduction

CSRF protection stops bad websites from tricking your site into doing things you don't want. It keeps your users safe.

When you have forms that change user data like passwords or emails.

When users can perform actions like posting comments or making purchases.

When you want to keep user sessions secure from outside attacks.

When your site uses cookies to remember users.

When you want to follow web security best practices.

Syntax

Express

import csrf from 'csurf';
import cookieParser from 'cookie-parser';

const csrfProtection = csrf({ cookie: true });

app.use(cookieParser());
app.use(csrfProtection);

app.get('/form', (req, res) => {
  res.send(`<form action='/process' method='POST'>
    <input type='hidden' name='_csrf' value='${req.csrfToken()}' />
    <button type='submit'>Submit</button>
  </form>`);
});

app.post('/process', (req, res) => {
  res.send('Data is processed safely.');
});

The csurf middleware adds a secret token to each user's cookie.

You must include the token in your forms as a hidden input named _csrf.

Examples

This sets up CSRF protection using cookies to store the token.

Express

const csrfProtection = csrf({ cookie: true });

This applies the CSRF protection middleware to all routes.

Express

app.use(csrfProtection);

This adds the CSRF token to your form so the server can check it later.

Express

res.send(`<input type='hidden' name='_csrf' value='${req.csrfToken()}' />`);

Sample Program

This Express app uses the csurf middleware to protect a form from CSRF attacks. The token is stored in a cookie and added to the form as a hidden input. When the form is submitted, the server checks the token before processing.

Express

import express from 'express';
import csrf from 'csurf';
import cookieParser from 'cookie-parser';

const app = express();
const port = 3000;

app.use(cookieParser());
app.use(express.urlencoded({ extended: false }));

const csrfProtection = csrf({ cookie: true });

app.get('/form', csrfProtection, (req, res) => {
  res.send(`
    <h1>Submit Form</h1>
    <form action='/process' method='POST'>
      <input type='hidden' name='_csrf' value='${req.csrfToken()}' />
      <button type='submit'>Submit</button>
    </form>
  `);
});

app.post('/process', csrfProtection, (req, res) => {
  res.send('Data is processed safely.');
});

app.listen(port, () => {
  console.log(`Server running on http://localhost:${port}`);
});

Important Notes

Always use cookie-parser before csurf if you use cookie-based tokens.

If the CSRF token is missing or wrong, the server will reject the request with an error.

Remember to include the CSRF token in every form that changes data.

Summary

CSRF protection stops unwanted actions from other sites.

Use the csurf middleware with cookies or sessions.

Include the CSRF token in your forms as a hidden field.

Practice

1. What is the main purpose of CSRF protection in an Express app?

easy

A. To prevent unauthorized commands from being sent from other websites

B. To speed up the server response time

C. To encrypt user passwords

D. To log user activity on the server

Solution

1. Step 1: Understand CSRF meaning

   CSRF stands for Cross-Site Request Forgery, which tricks users into submitting unwanted actions.

2. Step 2: Identify CSRF protection goal

   Protection stops other sites from sending commands on behalf of a user without permission.

3. Final Answer:

   To prevent unauthorized commands from being sent from other websites -> Option A

4. Quick Check:

   CSRF protection = prevent unauthorized commands [OK]

Hint: CSRF stops fake requests from other sites [OK]

Common Mistakes:

• Confusing CSRF with password encryption
• Thinking it speeds up server
• Believing it logs user activity

2. Which of the following is the correct way to add CSRF protection middleware in Express using the csurf package?

easy

A. app.use(csurf({ cookie: true }))

B. app.use(csrf())

C. app.use(csrfProtection())

D. app.use(csrf({ session: false }))

Solution

1. Step 1: Recall csurf usage

   The csurf middleware is used as csurf({ cookie: true }) to enable cookie-based CSRF tokens.

2. Step 2: Check options correctness

   Options B, C, and D use wrong function names or invalid options.

3. Final Answer:

   app.use(csurf({ cookie: true })) -> Option A

4. Quick Check:

   Correct csurf syntax = app.use(csurf({ cookie: true })) [OK]

Hint: Use csurf with correct function and options [OK]

Common Mistakes:

• Using wrong function name like csrf()
• Missing the cookie option
• Passing invalid options

3. Given this Express route using csurf middleware, what will happen if the CSRF token is missing or invalid?

app.post('/submit', csurf({ cookie: true }), (req, res) => {
  res.send('Form submitted');
});

medium

A. The server redirects to the home page

B. The server responds with 'Form submitted' anyway

C. The server throws a 403 Forbidden error

D. The server crashes with an uncaught exception

Solution

1. Step 1: Understand csurf error handling

   If the CSRF token is missing or invalid, csurf middleware triggers an error with status 403 Forbidden.

2. Step 2: Check route behavior

   The route handler is not called; instead, Express sends a 403 error response.

3. Final Answer:

   The server throws a 403 Forbidden error -> Option C

4. Quick Check:

   Invalid CSRF token = 403 Forbidden error [OK]

Hint: Missing token causes 403 error, not success [OK]

Common Mistakes:

• Assuming form submits anyway
• Thinking server redirects automatically
• Believing server crashes

4. You added csurf middleware but your form keeps failing CSRF validation. Which of these is the most likely cause?

medium

A. You did not install the cookie-parser package

B. You used app.use(express.json()) before csurf()

C. You set cookie: false in csurf options

D. You forgot to include the CSRF token in the form as a hidden input

Solution

1. Step 1: Check form token inclusion

   CSRF protection requires the token to be sent with the form, usually as a hidden input field.

2. Step 2: Evaluate other options

   While cookie-parser is needed if using cookies, the most common cause is missing token in the form.

3. Final Answer:

   You forgot to include the CSRF token in the form as a hidden input -> Option D

4. Quick Check:

   Missing token in form = validation fails [OK]

Hint: Always add CSRF token hidden input in forms [OK]

Common Mistakes:

• Ignoring token in form fields
• Misordering middleware without reason
• Assuming cookie-parser always required

5. You want to protect an Express app using csurf with cookie-based tokens and render the token in a form. Which code snippet correctly sets up the middleware and passes the token to the template?

hard

A. app.use(csurf({ cookie: false })); app.get('/form', (req, res) => { res.render('form', { csrfToken: req.csrfToken() }); });

B. app.use(csurf({ cookie: true })); app.get('/form', (req, res) => { res.render('form', { csrfToken: req.csrfToken() }); });

C. app.use(csurf()); app.get('/form', (req, res) => { res.render('form', { csrfToken: req.csrfToken }); });

D. app.use(csurf({ cookie: true })); app.get('/form', (req, res) => { res.render('form', { csrfToken: req.csrfToken }); });

Solution

1. Step 1: Setup csurf with cookie option

   Use csurf({ cookie: true }) to enable cookie-based CSRF tokens.

2. Step 2: Call req.csrfToken() as a function

   To get the token string, call req.csrfToken(), not just reference the function.

3. Step 3: Pass token to template

   Pass the token as csrfToken in the render call for the form to use.

4. Final Answer:

   app.use(csurf({ cookie: true })); app.get('/form', (req, res) => { res.render('form', { csrfToken: req.csrfToken() }); }); -> Option B

5. Quick Check:

   Correct csurf setup + token call = app.use(csurf({ cookie: true })); app.get('/form', (req, res) => { res.render('form', { csrfToken: req.csrfToken() }); }); [OK]

Hint: Call req.csrfToken() and enable cookie option [OK]

Common Mistakes:

• Not calling req.csrfToken() as a function
• Using cookie: false when cookies are needed
• Passing function reference instead of token string