# Week 3 - CORS Exploitation

## Exploiting CORS Misconfigurations

In week 3 of the Web App Hacking Tips & Tricks series, we will be covering CORS misconfigurations.\
 \
When I first became a tester, this topic caused me a good bit of confusion. It took me some time to find a clear and concise summary of how this attack (and CORS in general) worked. Here is my attempt at a clean and simple, Step-By-Step overview:\
 \
1\.      Specify a domain in the ‘Origin’ header of an HTTP Request\
\
2\.      Check if the response contains ‘Access-control-allow-origin: domain’ or ‘Access-control-allow-origin: null’\
\
\---> This means we can access HTTP Responses from our attacker site. The ‘null’ origin means any site can access responses\
\
3\.      Check if the response contains the ‘Access-control-allow-credentials: true’ header\
\
\---> This means we can send cookies with an HTTP Request, allowing us to access authenticated pages as a user\
 \
If the above steps are true, then the application has a CORS misconfiguration and can be exploited through a XSS attack using XMLHTTPRequest() like below:\
 \
`<script>`

`var xhr = new XMLHttpRequest();`\
`xhr.withCredentials = true;`\
`xhr.onreadystatechange = function() {`\
&#x20;  `if (xhr.readyState == XMLHttpRequest.DONE) {`\
&#x20;      `alert(xhr.responseText);`\
&#x20;  `}`\
`}`\
[`xhr.open`](http://xhr.open)`('GET', 'http://domain', true);`\
`xhr.send(null);`

`</script>`\
&#x20;\
An attacker can host this JavaScript payload on their domain, convince a user to click on their site, execute an HTTP request on their behalf, and then read the response.\
&#x20;

{% hint style="info" %}
It is important to note that if the vulnerable site uses a wildcard (‘Access-control-allow-origin: \*’) and allows credentials (Access-control-allow-credentials: true), it is NOT vulnerable. CORS blocks this by default, and will not send cookies with the request.
{% endhint %}

![](/files/DIbIO3K7ZYmeNnZlQ9HT)

![](/files/MVPpLnDWMPDgrT7bkCPs)


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://www.webhackingtips.com/weekly-tips/week-3-cors-exploitation.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.