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chrono

Challenge information

Level: Medium
Tags: picoCTF 2023, General Skills, linux
Author: MUBARAK MIKAIL

Description:
How to automate tasks to run at intervals on linux servers?

Use ssh to connect to this server:
Server: saturn.picoctf.net
Port: 57689
Username: picoplayer 
Password: KkPyI5bkmn

Hints:
(None)

Challenge link: https://play.picoctf.org/practice/challenge/347

Solution

Start by connecting to the server with SSH

┌──(kali㉿kali)-[/picoCTF/picoCTF_2023/General_Skills/chrono]
└─$ ssh -p 57689 [email protected]
The authenticity of host '[saturn.picoctf.net]:57689 ([13.59.203.175]:57689)' can't be established.
ED25519 key fingerprint is SHA256:p/PvzCEZdcZTX+VPBLVApO7dmZmo7L7qwjpiIdTTHao.
This host key is known by the following other names/addresses:
    ~/.ssh/known_hosts:1: [hashed name]
Are you sure you want to continue connecting (yes/no/[fingerprint])? yes
Warning: Permanently added '[saturn.picoctf.net]:57689' (ED25519) to the list of known hosts.
[email protected]'s password: 
Welcome to Ubuntu 20.04.5 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

This system has been minimized by removing packages and content that are
not required on a system that users do not log into.

To restore this content, you can run the 'unminimize' command.

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:~$

I know since previously that scheduled jobs in linux is specified in the crontab.
Otherwise this might require some googling to find out...

You can list your cronjobs with crontab -l so let's try that

picoplayer@challenge:~$ crontab -l
no crontab for picoplayer

Hm, no joy there. The configuration files for cron are, like most other configurations, stored in the /etc directory so let's go there

picoplayer@challenge:~$ cd /etc
picoplayer@challenge:/etc$ ls -la cron*
-rw-r--r-- 1 root root 43 Mar 16 02:00 crontab

cron.d:
total 8
drwxr-xr-x 1 root root  26 Mar 16 02:00 .
drwxr-xr-x 1 root root  66 Jul 25 17:20 ..
-rw-r--r-- 1 root root 102 Feb 13  2020 .placeholder
-rw-r--r-- 1 root root 201 Feb 14  2020 e2scrub_all

cron.daily:
total 12
drwxr-xr-x 1 root root   26 Mar 16 02:00 .
drwxr-xr-x 1 root root   66 Jul 25 17:20 ..
-rw-r--r-- 1 root root  102 Feb 13  2020 .placeholder
-rwxr-xr-x 1 root root 1478 Apr  9  2020 apt-compat
-rwxr-xr-x 1 root root 1187 Sep  5  2019 dpkg

cron.hourly:
total 4
drwxr-xr-x 2 root root  26 Mar 16 02:00 .
drwxr-xr-x 1 root root  66 Jul 25 17:20 ..
-rw-r--r-- 1 root root 102 Feb 13  2020 .placeholder

cron.monthly:
total 4
drwxr-xr-x 2 root root  26 Mar 16 02:00 .
drwxr-xr-x 1 root root  66 Jul 25 17:20 ..
-rw-r--r-- 1 root root 102 Feb 13  2020 .placeholder

cron.weekly:
total 4
drwxr-xr-x 2 root root  26 Mar 16 02:00 .
drwxr-xr-x 1 root root  66 Jul 25 17:20 ..
-rw-r--r-- 1 root root 102 Feb 13  2020 .placeholder

Let's start by checking the /etc/crontab file which also contains the flag.

picoplayer@challenge:/etc$ cat crontab 
# picoCTF{<REDACTED>}

For additional information, please see the references below.

References