**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**, 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.md#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
## 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 tied to a non-session cookie
Some applications do tie the CSRF token to a cookie, but **not** to the same **cookie** that is used to track **sessions**. This can easily occur when an application employs two different frameworks, one for session handling and one for CSRF protection, which are not integrated together.
If the web site contains any **behaviour** that **allows an attacker to set a cookie in a victim's browser**, then an **attack** is possible.
### CSRF token is simply duplicated in a cookie
In a further variation on the preceding vulnerability, some applications do not maintain any server-side record of tokens that have been issued, but instead **duplicate each token within a cookie and a request parameter**. When the subsequent request is validated, the application simply verifies that the **token** submitted in the **request parameter matches** the value submitted in the **cookie**.
In this situation, the attacker can again perform a CSRF **attack if the web site contains any cookie setting functionality**.
According to [**this**](https://developer.mozilla.org/en-US/docs/Web/HTTP/CORS#simple_requests), n 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`**_._
If a **CSRF token** is being used as **defence** you could try to **ex-filtrate it** abusing a [**XSS**](xss-cross-site-scripting/#xss-stealing-csrf-tokens) vulnerability or a [**Dangling Markup**](dangling-markup-html-scriptless-injection.md) vulnerability.
### **Make a GET request using** _**img**_ **tag**
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>
```
### POST request using multipart/form-data content type
```javascript
myFormData = new FormData();
var blob = new Blob(["<?php phpinfo(); ?>"], { type: "text/text"});
The code can be used to Brut Force a login form using a CSRF token \(It's also using the header X-Forwarded-For to try to bypass a possible IP blacklisting\):
