Post

OverTheWire: Bandit Level 22 → Level 23

OverTheWire - Bandit

Level Goal

A program is running automatically at regular intervals from cron, the time-based job scheduler. Look in /etc/cron.d/ for the configuration and see what command is being executed.
NOTE: Looking at shell scripts written by other people is a very useful skill. The script for this level is intentionally made easy to read. If you are having problems understanding what it does, try executing it to see the debug information it prints.

Commands you may need to solve this level

cron, crontab, crontab(5) (use “man 5 crontab” to access this)

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> whatis cron  
cron (8)         - daemon to execute scheduled commands (Vixie Cron)

> whatis crontab  
crontab (1)      - maintain crontab files for individual users (Vixie Cron)  
crontab (5)      - tables for driving cron

Helpful Reading Material

Linux/Mac Tutorial: Cron Jobs - How to Schedule Commands with crontab - YouTube

Linux crontab command help and examples

11 Cron Scheduling Task Examples in Linux

Solution

Since we know there is a task that is being executed by cron let’s have a look at all the cron jobs on the system

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bandit22@bandit:~$ ls /etc/cron.d/  
cronjob_bandit15_root  cronjob_bandit22  cronjob_bandit24  
cronjob_bandit17_root  cronjob_bandit23  cronjob_bandit25_root

Since we require the password for bandit23 the task that we are looking for should be cronjob_bandit23

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bandit22@bandit:~$ cat /etc/cron.d/cronjob_bandit23
@reboot bandit23 /usr/bin/cronjob_bandit23.sh  &> /dev/null
* * * * * bandit23 /usr/bin/cronjob_bandit23.sh  &> /dev/null

When we look at the cron job for bandit23 we see that there is a shell script that is being executed every second

(For detailed information on the syntax of cron jobs refer to the attached resources)

Let’s have a look at the contents of the script and try to understand what is it trying to perform

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bandit22@bandit:~$ cat /usr/bin/cronjob_bandit23.sh
#!/bin/bash

myname=$(whoami)
mytarget=$(echo I am user $myname | md5sum | cut -d ' ' -f 1)

echo "Copying passwordfile /etc/bandit_pass/$myname to /tmp/$mytarget"

cat /etc/bandit_pass/$myname > /tmp/$mytarget

Let’s break down the script line by line

  • The output of whoami command is getting saved in a variable called myname (Since this script is being executed for bandit23 the output of whoami will be bandit23 which is saved in the myname variable)
  • Next the sentence “I am user bandit23” is passed to the md5sum command which will calculate the md5sum of the given string. Lastly using the cut command the first field from the output of the md5sum command is selected and saved in the variable mytarget
  • Then a file is created in the /tmp directory with the name of the file being the same as the value of “mytarget”
  • And finally, the password of bandit23 is saved into that file

So looking at this script we can say that our goal is to find the value of “mytarget”. Since we know that the value of “myname” is bandit23 let’s see if we can able to generate the value of “mytarget”.

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bandit22@bandit:~$ echo "I am user bandit23" | md5sum | cut -d ' ' -f 1  
8ca319486bfbbc3663ea0fbe81326349

We have got the value of “mytarget” that the script creates for bandit23. Now that we have the value let’s get the password for bandit23

View the content of the file 8ca319486bfbbc3663ea0fbe81326349 that is present in the /tmp directory

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bandit22@bandit:~$ cat /tmp/8ca319486bfbbc3663ea0fbe81326349  
jc1udXuA1tiHqjIsL8yaapX5XIAI6i0n

And there we go we have the password for the next level !!!

Logout of the current session and start the next level as bandit23

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> ssh bandit23@bandit.labs.overthewire.org -p 2220
This is a OverTheWire game server. More information on http://www.overthewire.org/wargames

bandit23@bandit.labs.overthewire.org's password: jc1udXuA1tiHqjIsL8yaapX5XIAI6i0n
This post is licensed under CC BY 4.0 by the author.