[Write up] DEFCON CTF 2015 - wwtv , cybergrandsandbox


The bug is easy to find at function Coordidate, this is basic format string bug

But before you enter printf(s), we must to bypass the check a pair float number is parsed from s, to bypass it we just append format string bug to the end of the pair '51.492137,-0.192878' , for more information about atof read this http://www.cplusplus.com/reference/cstdlib/atof/

So the payload to exploit this bug too easy:

  1. First, we need to leak binary base address, and libc address
  2. Second, calc system address and then overwrite atof got by system address and then pwned.

But the game is not over, before we exploit the bug, we need to solve 2 problems:

We must to write a program to solve the game to enter TARDIS mode (this task is to quite strange)

We must bypass timecheck to enter vulnerable function

time_c > 0x55592B6C && time_c <= 0x55592B7F;

We must set time_c in range (0x55592b6c,0x55592b7f].

Take a look at READ_DATA function , will be triggered after 2 second.

OMG, the buffer was used for saving the connection to localtime server was used to store user input. We just send 9 zero bytes to server and then wait until READ_DATA is triggered and then send 4 bytes in require range, and we will enter vulnerable function.

Our poc here : https://gist.github.com/peternguyen93/f06aa5e27626598a1c21


This is very interesting challenge.

After doing RE we find some usefull information:

This program implementing basic Polish Notation by using JIT compiler.

The structure of jit is:

Take a look at function handle_digit 

When we inputted a string of number is seperated by space character , the jit compiter will push it in the stack_buffer.

We know that size of stack_buf is 0x1000 (located below stack_code), in this function there are no unbound checking if we push the stack_buf into stack_code, and so this bug does.

We just write own shellcode and then overwrite some opcode in the end of asm_code with own shellcode  (because cgc executable is not have sys_execve syscall so we just use some syscall provided by CGC to read the flag).

Our shellcode :

	push 0x3
	pop eax
	push ebx
	pop ecx
	push 0x3
	pop ebx
	push 0x50
	pop edx ;ebx hold my buffer
	int 0x80
	push 0x2
	pop eax
	push 0x1
	pop ebx
	int 0x80

Our poc is : https://gist.github.com/peternguyen93/e7d08cf109b38af6baae

This is the first time i wrote writeup using english, if something went wrong or some point you dont understand, feel free to ask me above

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