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**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**.
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.
* [**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._
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**.
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.
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.
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**_
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**.
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.
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:
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`**_._
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.\
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).
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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.md) vulnerability.
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)
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):
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