Let’s say you are operating an affiliate system where people can earn a commission for referring customers. Once a certain amount has been accumulated, users of your affiliate system can request a payout to their bank account.
Now, one day you get a rapidly increasing amount of support requests that money is disappearing from the affiliate system. After some investigation you find out that users have initiated a payout and the money has been transferred. When confronted with this, your users deny having done so.
What happened? Have you been hacked? Have your users passwords been stolen? Alarm bells start ringing. Everyone on deck, all developers feverishly dig through the logs, but nothing crops up. No logins from strange IP addresses, nothing out of the ordinary. Simply your users initiating transfers out of your system from their machine.
Your developers are stumped. How can this be? The number of complaints is too large to be a coincidence. At a loss you start poking around, and in one of the forums your users regularly converse you find a strange post. It consists of nothing but an image, but the image is broken. It doesn’t load. Coincidentally, the post was made at the exact time the strange transfers started happening.
That’s more than strange, so you start to investigate. Right click, inspect. Oddly the image has a very familiar URL:
Say hi to CSRF
This is a true story, I’ve seen this happen. Not exactly as described, but it happened nonetheless. But what exactly happened in the background?
Well, what you see is the simplest form of a Cross-Site Request Forgery or CSRF. Every time someone, who was logged in to your system, visited the forum, the browser would try to load the URL. Your site would receive the request and process it as a legitimate request from the user and send back a webpage. The browser, of course, would not be able to interpret the webpage as an image, which is why the image looked broken when you looked at it, but that doesn’t matter. The request was sent and your system made the transfer with the users normal login session.
Defense attempt #1: change it to POST
Your developers, of course, immediately know the answer: let’s change the transfer page so it only accepts
GET. That way when the browser sends a
GET for the image, nothing would happen.
Yes, it stops this simple attack, but it won’t stop all forms of CSRF. Let’s say the attacker builds a simple webpage they lure their victims to. The code is as follows:
<html> <body> <form action="https://yoursite.com/transfers/out" method="POST"> <input type="hidden" name="to" value="attackers-bank-account-number" /> <input type="hidden" name="amount" value="99999" /> </form> <script> document .getElementsByTagName("form") .submit(); </script> </body> </html>
As you can see, simply switching the transport methods will not solve the problem.
Mounting a proper defense: tokens
Let’s find a solution that is actually safe. We do that by requiring the form to include a secret token. This token is unique to the login session of the user and cannot be read by the attacker.
Our proper form would look something like this:
<form action="https://yoursite.com/transfers/out" method="POST"> <input type="text" name="to" value="" /> <input type="number" name="amount" value="" /> <input type="hidden" name="csrf-token" value="super-secret-value" /> </form>
Now, when the form is submitted, the server side compares the submitted token with the token stored in the users session on the server side. If it matches, the transfer is let through. If not, the transfer is rejected.
Note: It is very important that the token be unique to the users login session so that an attacker cannot get a hold of it.
Alternatively we could also store the CSRF token in a cookie with the users as the attacker will not be able to get a hold of that without some further vulnerability on your side.
Single-Page Apps are not immune
You may, of course, think that you are immune to this type of attack if you use a fancy Single-Page App (SPA) and communicate with the server using APIs.
CSRF protection is important
CSRF protection is just as important as preventing injection-type attacks. Don’t neglect it, not even in seemingly innocuous places like a login form. ∎