I'll be adding more challenges on this writeup as I do more , but basically pwnable.kr is a website which offers practice on binexp (binary exploitation / pwn) challenges , the have challenges based in different levels !

### First Challenge : FD

Mommy! what is a file descriptor in Linux?

* try to play the wargame your self but if you are ABSOLUTE beginner, follow this tutorial link:
https://youtu.be/971eZhMHQQw

ssh [e-mail] -p2222 (pw:guest)

Sounds fairly easy , so it's basically about file descriptors!, so let's connect and take a look on the challeng:

fd@pwnable:~$ ls
fd  fd.c  flag

So we got three files, where as `fd` is our binary, and the `flag` which contains our flag, and `fd.c` which is the source code of `fd`

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
char buf[32];
int main(int argc, char* argv[], char* envp[]){
	if(argc<2){
		printf("pass argv[1] a number\n");
		return 0;
	}
	int fd = atoi( argv[1] ) - 0x1234;
	int len = 0;
	len = read(fd, buf, 32);
	if(!strcmp("LETMEWIN\n", buf)){
		printf("good job :)\n");
		system("/bin/cat flag");
		exit(0);
	}
	printf("learn about Linux file IO\n");
	return 0;

}

So we break down the source code to understand what it does and it looks like it takes an argument which is to be an input and then we got `int fd = atoi(argv[1]) - 0x1234` which means the input you gave in will be converted to int, then `read(fd,buf,32)` where the buf is assigned to buffer of 32 characters.

So let's get back to school we have 3 types of file descriptors:

stdin with int value(0)
stdout with int value(1)
stderr with int value (2)

So first thing I do is to know the int value of 0x1234 which is being substracted to `argv[1]`

└──╼ $python3 
Python 3.9.2 (default, Feb 28 2021, 17:03:44) 
[GCC 10.2.1 20210110] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> 0x1234
4660

okay basically `0x1234` = `4660`, so what it does watever input we put in will be substracted with `0x1234` then it'll take our input and then checks if buf is equal with `LETMEWIN` and if it is , it'll give us the flag , if not then we can't read the flag it'll print `learn about Linux file IO`, Basically we have to make `argv[1] - 0x1234 = 0` so as stdin is specified and then we can be able to input `buf`:

fd@pwnable:~$ ./fd 4660
LETMEWIN
good job :)
mommy! I think I know what a file descriptor is!!

FLAG : `mommy! I think I know what a file descriptor is!!`

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### WILL CONTINUE WHEN I DO THE NEXT CHALLENGE