Given Code Snippet:

Code review:

easy-eval.js

if (window.debug) {
    eval(window.debug.toString()); //using eval at DOM element with id "debug"
  //only a and area tag can be used in attack as they are capable of using href attribute. toString get only that attribute
}

easy-xss.js

const pos = document.URL.indexOf('name=') + 5; //user input
const name = document.URL.substring(pos, document.URL.length)// just paring GET parameteres
const container = document.getElementById('container');
container.innerHTML = decodeURI(name); // no proper sanitization

index.html

//can't use inline script tag because it has to be src "self"
<meta http-equiv="Content-Security-Policy" content="default-src 'self'; script-src 'self' 'unsafe-eval'"> 
<div id="container"></div>
<script src="easy-eval.js"></script>
<script src="easy-xss.js"></script>

The Vulnerability

User input is passed in ‘name’ parameter straight to the ‘innerHTML’ so it would be rendered by browser for example inserting ‘<h1>asdf</h1>’ makes ‘asdf’ bold in browser, so ‘HTML’ tag is parsed correctly.

Exploitation

Here, The application’s CSP — Content Security Policy is not enough to stop executing arbitary JavaScript code.

Using iframe with srcdoc attribute allows to fullfil defaul-src: self condition of CSP. This is because iframe with srcdoc is assumed src= self. This with addition to no proper sanitization allows to inject JavaScript code in the victim browser.

Payload

http://127.0.0.1:8000/?name=<iframe srcdoc="<a id=debug href=pb:alert(document.domain)><script src=easy-eval.js></script>">

The srcdoc attriburte inject a tag with id=debug which allows to pass if statement in easy-eval.js then href attribute is set to pb:alert(document.domain) the first part pb should be nonexisting protocol. Any protocol that contains // would not work because in javascript // is comment. So http:// or ftp:// would result in commenting the payload and never executing it properly.

After that easy-eval.js is called again to reinitalize the script and execute code in it.

XSS

Paweł Wąsik and I worked together to understand and identify the JS code and the vulnerability respectively. This challenge seems to be quite interesting and we were able to gain new knowledge from it.

Thanks Richard for providing an excellent explanation that greatly contributed to our understanding of the subject.

Refer to this thread to gain a better understanding.

https://twitter.com/h43z/status/1626237041787940867?s=20

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Twitter — Prashanth, Pawel