CSRF Attacks Explained
How They Work and How to Prevent Them
Cross-Site Request Forgery (CSRF) is one of the most overlooked yet dangerous web vulnerabilities, often exploited in applications where user sessions are poorly protected. At its core, CSRF tricks an authenticated user into performing unintended actions on a web application, without their knowledge or consent. For fintech systems, banking apps, or any platform handling sensitive financial transactions, understanding the anatomy of CSRF is critical for building secure products.
How a CSRF Attack Works
The fundamental principle of a CSRF attack lies in session hijacking through implicit trust. If a user is authenticated on a site—say, their online banking portal—their browser automatically includes session cookies in every request. Attackers exploit this by luring the user into making a malicious request (for example, by clicking a hidden link or loading an image on another website).
For example:
1 
If the victim is logged in, their browser may automatically send the banking session cookie along with this request. Without proper CSRF protections in place, the bank might process the transfer as if it were legitimate.
Anatomy of the Attack
Victim Authentication – The user logs into a trusted service (e.g., their bank).
Malicious Trigger – The attacker sends the victim a crafted URL or embeds a request in a malicious webpage.
Automatic Request – The browser includes authentication cookies in the request, since the victim is logged in.
Unintended Action – The server executes the request, such as changing a password or transferring money.
The most dangerous aspect: the victim is unaware of the fraudulent action until it’s too late.
Why CSRF is Critical in Fintech
For financial applications, CSRF isn’t just about inconvenience—it’s about fraud, fund transfers, and reputation damage. Unlike simple injection attacks, CSRF exploits the implicit trust between a user and their session, making it particularly challenging to detect after the fact. Regulators like the European Banking Authority (EBA) under PSD2 demand strict transaction-level protections, which means CSRF countermeasures aren’t optional for fintech platforms.
Prevention Strategies
1. Anti-CSRF Tokens
The most widely used defense involves adding unique, unpredictable tokens to sensitive requests. These tokens (often called synchronizer tokens) are validated by the server, ensuring the request originates from the legitimate session.
Example implementation in an Express.js app:
1 // Using csurf middleware
2 const csrf = require('csurf');
3 const csrfProtection = csrf({ cookie: true });
4 app.use(csrfProtection);
5
6 app.get('/transfer', (req, res) => {
7 res.render('transfer', { csrfToken: req.csrfToken() });
8 }); 2. SameSite Cookies
Modern browsers support the SameSite cookie attribute, which restricts cookies from being sent with cross-site requests. Setting it to Strict or Lax drastically reduces CSRF risk.
1 Set-Cookie: sessionId=abc123; HttpOnly; Secure; SameSite=Strict 3. Re-Authentication for Sensitive Actions
Fintech systems often require re-authentication (e.g., entering a PIN, SMS OTP, or biometric) for critical operations like fund transfers. This neutralizes CSRF attempts since the attacker cannot bypass the second-factor check.
4. Double-Submit Cookie Pattern
Another strategy is sending a CSRF token both in a cookie and in the request body/headers. If they don’t match, the request is rejected.
5. Security-Aware UI Patterns
Embedding confirmation dialogs, transaction details, and verification flows ensures users are consciously approving actions, reducing blind execution of forged requests.
Conclusion
CSRF attacks thrive in environments where user sessions and request integrity are poorly managed. For fintech and banking applications, these vulnerabilities pose significant financial and reputational risks. By adopting layered defenses—ranging from anti-CSRF tokens and SameSite cookies to re-authentication and secure UX patterns—development teams can protect customers and meet regulatory standards.
CSRF may be an old vulnerability, but in high-stakes industries like finance, it’s far from obsolete.
