![alt chal_desc](./chall_desc.png)

### Author: p4w
### Twitter: @p4w16

Running the binary:
$ ./unary
1. x++
2. x--
3. ~x
4. -x
Operator: 1
x = 5
f(x) = 6
1. x++
2. x--
3. ~x
4. -x

Disassemble of main():
0x0040080f call show_menu ; sym.show_menu
0x00400814 mov rdi, rbp
0x00400817 call read_int ; sym.read_int
0x0040081c mov ebx, eax
0x0040081e test eax, eax
0x00400820 je 0x400855
0x00400822 mov rdi, r14
0x00400825 call read_int ; sym.read_int
0x0040082a sub ebx, 1
0x0040082d movsxd rbx, ebx
0x00400830 mov rsi, r13
0x00400833 mov edi, eax
0x00400835 call qword [r12 + rbx*8]
Disassemble of read_int():
0x0040079e sub rsp, 0x18
0x004007a2 mov rdx, rdi ; arg1
0x004007a5 lea rsi, [0x00400916]
0x004007ac mov edi, 1
0x004007b1 mov eax, 0
0x004007b6 call __printf_chk ; sym.imp.__printf_chk
0x004007bb lea rsi, [var_ch]
0x004007c0 lea rdi, [0x00400919] ; const char *format
0x004007c7 mov eax, 0
0x004007cc call __isoc99_scanf ; sym.imp.__isoc99_scanf ; int scanf(const char *format)
0x004007d1 cmp eax, 1 ; rdx
0x004007d4 je 0x4007e0
0x004007d6 mov edi, 0 ; int status
0x004007db call exit ; sym.imp.exit ; void exit(int status)
0x004007e0 mov eax, dword [var_ch]
0x004007e4 add rsp, 0x18
0x004007e8 ret

The first call to `read_int()` read our input using `__isoc99_scanf()` and the rexult got stored in `eax` (0x004007e0), after that the contents of `eax` got transferred into `ebx` (0x0040081c).
The vulnerable line reside in 0x00400835 of `main()` func, as we can see it does a `call qword [r12 + rbx*8]` and we can control the `rbx` register with our input (our input is used as an offsett to a function pointer).
The sencond input we can pass to the program, will be the argument to the func. pointer (look at instruction addr. 0x00400833).

How we can abuse this behavior?

Passing an offsett (our first input) and trick the programm to make a call instruction on __puts@GOT__ to leak libc address, then use same method to call ____isoc99_scanf@GOT__ with a `"%s"` argument to gain stack buffer overflow. From there we can just use __ROP__ to gain a shell.

The exploit is available here

Original writeup (https://github.com/beerpwn/ctf/tree/master/2020/SuSeC_CTF/pwn/unary).