CSRF protection in Express - Step-by-Step Execution

Concept Flow - CSRF protection

Client sends request

↓

Server checks CSRF token

↓

Process

↓

Send response

The server checks if the CSRF token sent by the client matches the expected token before processing the request.

Execution Sample

Express

const cookieParser = require('cookie-parser');
const csrf = require('csurf');
const express = require('express');
const app = express();
app.use(cookieParser());
app.use(express.urlencoded({ extended: false }));
app.use(csrf({ cookie: true }));
app.get('/form', (req, res) => {
  res.send(`<form method="POST" action="/form">
    <input type="hidden" name="_csrf" value="${req.csrfToken()}">
    <button type="submit">Submit</button>
  </form>`);
});
app.post('/form', (req, res) => {
  res.send('Form submitted');
});

This code adds CSRF protection middleware to an Express app and handles a POST form submission.

Execution Table

Step	Request Type	CSRF Token Present	Token Valid?	Action	Response
1	GET /form	No (token generated)	N/A	Generate token and send form	Form with CSRF token
2	POST /form	Yes (token sent)	Yes	Process form data	Form submitted
3	POST /form	Yes (token sent)	No	Reject request	403 Forbidden
4	POST /form	No token	No	Reject request	403 Forbidden

💡 Requests without a valid CSRF token are rejected to prevent CSRF attacks.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4
csrfToken	undefined	generated token string	token string from client	token string from client	undefined

Key Moments - 2 Insights

Why does the server reject a POST request without a CSRF token?

Because the CSRF middleware requires a valid token to confirm the request is from the trusted client, as shown in execution_table rows 3 and 4.

When is the CSRF token generated and sent to the client?

During a safe GET request, the server generates and sends the token embedded in the form, as shown in execution_table row 1.

Visual Quiz - 3 Questions

Look at the execution table, what happens at step 2 when the token is valid?

A The server generates a new CSRF token

B The server rejects the request with 403 Forbidden

C The server processes the form data and sends a success response

D The server ignores the token and processes the request

💡 Hint

Refer to execution_table row 2 under 'Action' and 'Response'

At which step does the server generate the CSRF token?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Check execution_table row 1 where the token is generated and sent

If the client sends a POST request without any CSRF token, what response will the server give?

A 200 OK with form submitted message

B 403 Forbidden error

C Redirect to GET /form

D 500 Internal Server Error

💡 Hint

See execution_table row 4 where no token causes rejection

Concept Snapshot

CSRF protection in Express:
- Use csurf middleware to add CSRF tokens
- Server generates token on safe requests (GET)
- Client sends token with unsafe requests (POST)
- Server validates token before processing
- Reject requests with missing or invalid tokens
- Prevents unauthorized cross-site requests

Full Transcript

CSRF protection in Express works by generating a unique token for each user session during safe requests like GET. This token is sent to the client embedded in forms. When the client submits a form with a POST request, it must include this token. The server checks the token's validity before processing the request. If the token is missing or invalid, the server rejects the request with a 403 Forbidden error. This process prevents attackers from tricking users into submitting unwanted requests from other sites. The csurf middleware handles token generation and validation automatically. The flow starts with the client requesting a form, the server generating and sending a token, then the client submitting the form with the token, and the server validating it before accepting the data.