picoCTF 2023 – useless

28 March 2023 – Written by Valentin Huber – in ctf, grep, and man

Challenge

There’s an interesting script in the user’s home directory

Script content:

#!/bin/bash
# Basic mathematical operations via command-line arguments

if [ $# != 3 ]
then
  echo "Read the code first"
else
	if [[ "$1" == "add" ]]
	then 
	  sum=$(( $2 + $3 ))
	  echo "The Sum is: $sum"  

	elif [[ "$1" == "sub" ]]
	then 
	  sub=$(( $2 - $3 ))
	  echo "The Substract is: $sub" 

	elif [[ "$1" == "div" ]]
	then 
	  div=$(( $2 / $3 ))
	  echo "The quotient is: $div" 

	elif [[ "$1" == "mul" ]]
	then
	  mul=$(( $2 * $3 ))
	  echo "The product is: $mul" 

	else
	  echo "Read the manual"
	 
	fi
fi

Solution

The Read the manual line prompted me to do so:

man useless

useless
     useless, — This is a simple calculator script

SYNOPSIS
     useless, [add sub mul div] number1 number2

DESCRIPTION
     Use the useless, macro to make simple calulations like addition,subtraction, multiplication and di‐
     vision.

Examples
     ./useless add 1 2
       This will add 1 and 2 and return 3

     ./useless mul 2 3
       This will return 6 as a product of 2 and 3

     ./useless div 6 3
       This will return 2 as a quotient of 6 and 3

     ./useless sub 6 5
       This will return 1 as a remainder of substraction of 5 from 6

Authors
     This script was designed and developed by Cylab Africa

     picoCTF{us3l3ss_ch4ll3ng3_3xpl0it3d_1155}