Check that **targetOrigin** could be a url like _https://company.com_, so the messages can only be sent to that user \(secure\). Or it cloud be a wildcard "**\***". In case a wildcard is used, messages could be sent to any domain.
### Attack
In [**this report**](https://blog.geekycat.in/google-vrp-hijacking-your-screenshots/) you can read how you could **iframe a page** that at some point may **sent** a **postmessage** using a **wildcard as targetOrigin** and **modify it's location so the data will be sent to an arbitrary domain**. In order to be able to perform this attack **X-Frame header must not be present** in the vuln page.
In order to treat the messages a code similar to the following one will be used:
```javascript
window.addEventListener("message", (event) => {
if (event.origin !== "http://example.org:8080")
return;
// ...
}, false);
```
Note in this case how the first thing that the code is doing is **checking the origin**. This is terribly **important** mainly if the page is going to do **anything sensitive** with the received information \(like changing a password\). If it doesn't check the origin, attackers can make victims send arbitrary data to this endpoints and change the victims passwords \(in this example\).
It's important to check the origin and it's **equally important to check it right:**
Copied from [https://jlajara.gitlab.io/web/2020/07/17/Dom\_XSS\_PostMessage\_2.html](https://jlajara.gitlab.io/web/2020/07/17/Dom_XSS_PostMessage_2.html)
* If `indexOf()` is used to check the origin of the PostMessage event, remember that it can be bypassed if the origin is contained in the string as seen in [_The Bypass_](https://jlajara.gitlab.io/web/2020/07/17/Dom_XSS_PostMessage_2.html#bypass)
* [@filedescriptor](https://twitter.com/filedescriptor): Using `search()` to validate the origin could be insecure. According to the docs of `String.prototype.search()`, the method takes a regular repression object instead of a string. If anything other than regexp is passed, it will get implicitly converted into a regexp.
```javascript
"https://www.safedomain.com".search(t.origin)
```
In regular expression, a dot \(.\) is treated as a wildcard. In other words, any character of the origin can be replaced with a dot. An attacker can take advantage of it and use a special domain instead of the official one to bypass the validation, such as **www.s.afedomain.com**.
* [@bored-engineer](https://bored.engineer/): If `escapeHtml` function is used, the function does not create a `new` escaped object, instead it over-writes properties of the existing object. This means that if we are able to create an object with a controlled property that does not respond to `hasOwnProperty` it will not be escaped.
```javascript
// Expected to fail:
result = u({
message: "'\"<b>\\"
});
result.message // "'"<b>\"
// Bypassed:
result = u(new Error("'\"<b>\\"));
result.message; // "'"<b>\"
```
`File` object is perfect for this exploit as it has a read-only `name` property which is used by our template and will bypass `escapeHtml` function.
