useless

Challenge Description

This challenge requires us to first connect to their SSH server as picoplayer on port 59574. We can do this by running ssh -p 59574 picoplayer@saturn.picoctf.net. The -p argument is used to specify the port.

After successfully connecting to the server, I ran ls to check the files and directories within the home directory (~).

As expected, there’s a file named useless within this directory. In fact, it was the only file in this directory. Running file useless shows that it is a Borne-Again shell (bash) script.

I proceeded to read the script:

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

At the first glance, it seemed like the script did not contain any clue for us to proceed, However, upon closer inspection, we see that there’s a part that requires our attention:

else
    echo "Read the manual"

Manual pages in Linux

In order to read manual pages of commands in Linux, we can run the man command. This is the basic syntax of it:

man [option] [command]

t provides detailed documentation about the usage, options, and functionality of commands, making it an essential tool for both beginners and experienced users (GeeksforGeeks, 2024).

”Useless” manual page?

The following output is displayed when I ran man useless:

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_8504}
 

There it is! The flag is included in the output.

Flag

picoCTF{us3l3ss_ch4ll3ng3_3xpl0it3d_8504}

References

• GeeksforGeeks. (2024d, June 10). How to Read Manual Pages in Linux | man Command. GeeksforGeeks. https://www.geeksforgeeks.org/man-command-in-linux-with-examples/