# 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 **`![]()
`** (note that Sanitizer API -[here](https://wicg.github.io/sanitizer-api/index.html#dom-clobbering)- is not configured to protect against DOM clobbering attacks in its default state).
- Therefore, you can clobber **`document.getElementById("calc")`** with **`![]()
`**. 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
```
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