useless

• Description: There's an interesting script in the user's home directory
• Difficulty: Medium

---

🔎 Solution

After gaining SSH access to the target machine, we noticed a file named useless located in the user's directory.

useless: Bourne-Again shell script, ASCII text executable

Viewing the file's content, we found that it was a simple Bash script designed to perform basic arithmetic operations - addition, subtraction, multiplication, and division - based on command-line arguments.

picoplayer@challenge:~$ cat useless 
#!/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

Running the script with appropriate parameters yields the expected results of the corresponding arithmetic operation. For instance:

picoplayer@challenge:~$ bash useless add 2 3
The Sum is: 5
picoplayer@challenge:~$ bash useless mul 2 4
The product is: 8

At first glance, the script appears straightforward, with no obvious vulnerabilities or backdoors. However, upon closer inspection, we noticed that when the script is executed without exactly 3 arguments or with an unsupported operation, it returns a curious message: "Read the manual". This is a subtle but interesting hint. In Unix-like systems, the man (manual) command is commonly used to display documentation about commands, including usage, options, and examples. Developers often include a manual page for custom tools or scripts, especially if they want to hide information in plain sight.

So, following the clue, we tried man useless, and that's when we found the flag cleverly hidden in the manual page.

picoplayer@challenge:~$ 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 division.

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_6140}

🚩Flag

picoCTF{us3l3ss_ch4ll3ng3_3xpl0it3d_6140}