DEF CON CTF Qualifier 2021

Team Placing: #93 / 433

Solvers: This challenge and writeup was a collaborative effort between myself and my teammate, Dayton Hasty (dayt0n)

Challenge: nooopster

Category: PWN, RE


Sharing files like its 1999. Connect to my network with OpenVPN. 1999


We’re provided an OpenVPN config file and key:

dev tun
secret openvpn.shared.key
remote 1999 tcp-client
verb 3

Initially the config file contained the wrong remote IP so we were having trouble connecting but with some sharp eyes from Dayton, we were able to replace the incorrect IP with 1999. This was eventually fixed by the challenge authors.

Using OpenVPN we can connect like so:

sudo openvpn nooopster.ovpn

Based off the OpenVPN config file we know that our local VPN endpoint is and the remote VPN endpoint is So let’s try running a nmap scan on

$ sudo nmap -sC -sV -oN nmap/initial

7070/tcp open  napster         MLDonkey multi-network P2P client
8888/tcp open  sun-answerbook?

Because nmap told us that this was a MLDonkey server, we initially tried connecting via netcat to see if we could supply MLDonkey commands; as well as downloading MLDonkey and other eDonkey clients. All of which didn’t work for us.

After some time, we took a closer look at port 8888 and realized that it was actually the standard port for Napster which made a lot more sense given the challenge name nooopster.

Doing some research we were able to find a linux napster client called Nap.

Downloading the i386 Debian package and installing like so:

$ sudo dpkg --add-architecture i386

$ sudo apt-get update

$ sudo dpkg -i nap_1.5.2-2_i386.deb

We can now connect to the Napster server like so nap -s

After reading some documentation we were able to list the channels with the /clist command which revealed a channel called #chat. Then we connected to the channel:

/join #chat

There appeared to be a bot that repeatedly told us to check out his files. So let’s do that! We can use the /browse nooopster command to see what files he has:

Nice, that’s a lot of 90s songs (which are actually all rickrolls). But if you continue scrolling there’s one interesting file at the very end:

So let’s download it and run file:

$ file nooopster

nooopster: ELF 64-bit LSB pie executable, x86-64, version 1 (SYSV), dynamically linked, interpreter /lib64/, BuildID[sha1]=35b1628f65a3b5d518bc802d5cbe122bee0bf226, for GNU/Linux 3.2.0, stripped

Since this is an ELF binary, we will throw it into Binary Ninja and check out main() (functions have been renamed for readability purposes):

main function

Knowing that napster is a peer-to-peer file sharing service, the chdir("shared") call lead us to believe that this was the binary for the client the nooopster user was using.

It appears as though the initial connection is done with the first do_connect() and send/recv_nap_msg() call sequences.

Let’s check out that pthread_create() call:


It appears that a new thread is being created that executes the start_routine_1a80() function, which starts out by creating a new socket and creating a new thread to handle the connections to the server:


Now, we can take a look at the start_routine() function in the above pthread_create():


After digging around in the napster protocol specification, we are able to get a better idea of what is going on here. This above code appears to be for use in handling file download requests. As the protocol specification states in section 5.1 Normal Downloading, for each download request, a ‘GET’ string is sent. The code appears to check for that ‘GET’ string before processing the rest of the message.

Scrolling down into the function, we find a simple logic vulnerability in this program:


After receiving the filename via the ‘GET’ command, the function checks to make sure the file path starts with \shared\. Then, since it assumes it is running in the \shared\ directory already, it removes the \shared\ portion of the path and processes everything after it as a file of its own.

So, when we asked for the nooopster binary in the napster client, it sent a request to the vulnerable client that looked something like this:

GETdayt0n "\shared\nooopster" 0

We can take advantage of the fact that the vulnerable client processes everything after the \shared\ string and download virtually any file on the remote client’s system. To do this, we would need to send a request that looked like:

GETdayt0n "\shared\/etc/passwd" 0

This way, the program will attempt to open, read, and send the contents of the file at /etc/passwd.

The tricky part was actually triggering this bug in the first place.

It appeared that in order for us to send our exploit and retrieve the flag, we not only had to connect to the Napster server, but we also had to first download a file manually through the Napster, such as the nooopster binary from before. We made a script to ensure that we were able to send requests to the server after the manual download:

from pwn import *

p = remote('',7070)
p.sendline(r'GETducky "\shared\nooopster" 0')

Once we have confirmed that the server has responded to our requests, we just have to update the script to grab /flag:

from pwn import *

p = remote('',7070)
p.sendline(r'GETducky "\shared\/flag" 0')

Running the script, we successfully retrieve the flag!

Flag: OOO{M4573R_0F_PUPP375}