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useless.md

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useless

Challenge information

Level: Medium
Tags: picoCTF 2023, General Skills, man
Author: LOIC SHEMA

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

The work computer is running SSH. We've been given a script which performs some basic calculations, 
explore the script and find a flag.
 
Hostname: saturn.picoctf.net
Port:     55661
Username: picoplayer
Password: password
 
Hints:
(None)

Challenge link: https://play.picoctf.org/practice/challenge/384

Solution

Start by connecting to the server with SSH

┌──(kali㉿kali)-[/picoCTF/picoCTF_2023/General_Skills/useless]
└─$ ssh -p 55661 picoplayer@saturn.picoctf.net
The authenticity of host '[saturn.picoctf.net]:55661 ([13.59.203.175]:55661)' can't be established.
ED25519 key fingerprint is SHA256:ves7M6DhshpiJSsScBWo3n34oOFTUXvLZqPyqLWeTHk.
This key is not known by any other names.
Are you sure you want to continue connecting (yes/no/[fingerprint])? yes
Warning: Permanently added '[saturn.picoctf.net]:55661' (ED25519) to the list of known hosts.
picoplayer@saturn.picoctf.net's password: 
Welcome to Ubuntu 20.04.6 LTS (GNU/Linux 5.19.0-1024-aws x86_64)

 * Documentation:  https://help.ubuntu.com
 * Management:     https://landscape.canonical.com
 * Support:        https://ubuntu.com/advantage

The programs included with the Ubuntu system are free software;
the exact distribution terms for each program are described in the
individual files in /usr/share/doc/*/copyright.

Ubuntu comes with ABSOLUTELY NO WARRANTY, to the extent permitted by
applicable law.

picoplayer@challenge:~$

The script should be located in our home directory so let's look for it

picoplayer@challenge:~$ ls -la
total 16
drwxr-xr-x 1 picoplayer picoplayer   20 Jul 28 14:39 .
drwxr-xr-x 1 root       root         24 Mar 16 02:30 ..
-rw-r--r-- 1 picoplayer picoplayer  220 Feb 25  2020 .bash_logout
-rw-r--r-- 1 picoplayer picoplayer 3771 Feb 25  2020 .bashrc
drwx------ 2 picoplayer picoplayer   34 Jul 28 14:39 .cache
-rw-r--r-- 1 picoplayer picoplayer  807 Feb 25  2020 .profile
-rwxr-xr-x 1 root       root        517 Mar 16 01:30 useless

picoplayer@challenge:~$ file useless
useless: Bourne-Again shell script, ASCII text executable

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

Hhm, no flag there. But there is an instruction to read the manual and the challenge is also tagged with man so let's try that

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{<REDACTED>}

Ah, the flag is included at the bottom of the man page.

For additional information, please see the references below.

References