CSRF protection in Express - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

How does CSRF token validation work in Express?

Consider an Express app using the csurf middleware. What happens when a POST request is sent without the CSRF token?

Express

const express = require('express');
const csurf = require('csurf');
const cookieParser = require('cookie-parser');

const app = express();
app.use(cookieParser());
app.use(csurf({ cookie: true }));

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

A The server responds with a 403 Forbidden error because the CSRF token is missing or invalid.

B The server logs the request but still processes it without blocking.

C The server redirects the user to the login page automatically.

D The server accepts the request and processes it normally without any error.

💡 Hint

Think about what the csurf middleware does when the token is missing or wrong.

The csurf middleware checks for a valid CSRF token in the request. If the token is missing or invalid, it blocks the request by sending a 403 Forbidden error to prevent CSRF attacks.

📝 Syntax

intermediate

Which code snippet correctly sets up CSRF protection with cookies in Express?

Choose the correct code to enable CSRF protection using the csurf middleware with cookie support.

A app.use(csurf({ cookie: false }));

B app.use(csurf({ session: true }));

C app.use(csurf({ cookie: true }));

D app.use(csurf());

💡 Hint

Look for the option that enables cookie-based CSRF tokens.

To use cookie-based CSRF tokens, the csurf middleware must be configured with { cookie: true }. This tells csurf to store the token in a cookie instead of the session.

🔧 Debug

advanced

Why does the CSRF token validation fail in this Express app?

Given the code below, why does the server always respond with a 403 error on POST requests?

Express

const express = require('express');
const csurf = require('csurf');
const cookieParser = require('cookie-parser');

const app = express();
app.use(cookieParser());
app.use(csurf({ cookie: true }));

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

app.post('/submit', (req, res) => {
  res.send('Success');
});

A The app is missing express.urlencoded middleware before csurf, so the body is not parsed and token is not found.

B The cookieParser middleware is used after csurf, so cookies are not read correctly.

C The CSRF token is not included in the form, so validation fails.

D The csurf middleware is configured incorrectly; it should not use cookies.

💡 Hint

Check the order of middleware and what csurf needs to read the token from the body.

The csurf middleware requires the request body to be parsed before it can find the CSRF token. Without express.urlencoded middleware before csurf, the token in the form body is not accessible, causing validation to fail.

🧠 Conceptual

advanced

What is the main purpose of CSRF protection in web frameworks like Express?

Why do we add CSRF protection middleware in web applications?

A To authenticate users by verifying their passwords.

B To encrypt user data sent between client and server.

C To speed up server response times by caching requests.

D To prevent attackers from tricking users into submitting unwanted requests on their behalf.

💡 Hint

Think about what CSRF attacks do to users and servers.

CSRF protection stops attackers from making users unknowingly send harmful requests, like changing passwords or making purchases, by requiring a secret token that only the user’s browser knows.

❓ state_output

expert

What is the value of the CSRF token after this Express route runs?

Given the code below, what will be the output of the console.log statement?

Express

const express = require('express');
const csurf = require('csurf');
const cookieParser = require('cookie-parser');

const app = express();
app.use(cookieParser());
app.use(csurf({ cookie: true }));

app.get('/token', (req, res) => {
  const token = req.csrfToken();
  console.log(token);
  res.send('Token sent');
});

A An empty string because no token is generated.

B A unique string token generated for the user session, e.g., 'XyZ123AbC'.

C An error is thrown because cookieParser is missing.

D Undefined, because req.csrfToken() is not a function.

💡 Hint

What does req.csrfToken() return when csurf middleware is used correctly?

The csurf middleware adds the req.csrfToken() function which returns a unique token string for the current user session or cookie. This token is used to validate requests.