Security Programming Homework 2-1
- There is a vulnerability in piece of code which contains
- There is a
call eaxin the binary
- Because it is an interactive shell program, bad characters are 0x00, 0x0a, 0x2f (filter out by specific function in the program)
De-complie the binary
It is hard to debug using gdb. A possible method is to use gdbserver to do remote debugging. But sadly, I do not have time to learn how to use it. Maybe next time…
So I turned up using IDA Pro to do the static analysis. By decompilation, bunch of functions showed up:
... frame_dummy passCmdline splitCmdline parseArgv printEnv setEnv getResult sendResult closePipes exeCmd nextLine main passPipe addPipe delPipe ...
By doing some observation, I picked up three interesting functions for deeper
|Offset of cmdline||Meaning|
|0||pointer of whole command line|
|1 - 10000||pointer of each word|
|10001||number of words in whole command line|
|10002 - 11001||single command, seperated by ‘|’|
|11005||pointer of function printEnv()|
|11006||pointer of string “printenv”|
|11007||pointer of function setEnv()|
Since the program uses function pointer to call printEnv(), we can simply put our shellcode to replace the original printEnv() with the aid of buffer overflow.
According to the table above, stack overflow will happen when we type more than 1000 words in one single command (commands are seperated by ‘|’). To overwrite index 11005 with our shellcode, we need to stuff 1003 words in total.
The 1005th word will overwrite the pointer of the string “printenv”, which used to compared to the very first word. When they are equal, the program will run into printEnv(). So our job is to ensure they will be the same, otherwise it won’t call our shellcode.
The shellcode for execute
/bin/sh is showed following, grab from the
Internet. See the references at the end of this article for more information.
The first word should be the same as the last word, and the shellcode should be 1004th word. That’s all!
payload = 'a ' * 1003 payload += 'PYj0X40PPPPQPaJRX4Dj0YIIIII0DN0RX502A05r9sOPTY01A01RX500D05cFZBPTY01SX540D05ZFXbPTYA01A01SX50A005XnRYPSX5AA005nnCXPSX5AA005plbXPTYA01Tx ' payload += 'a'
Don’t forget to append a new line character to fire our payload!
The flag is