Writeups for Google CTF 2019: My first CTF (Kinda) - Part 2

Work Computer - Sandbox

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We connect to the given server on the given port and see what we are up against by using netcat (nc readme.ctfcompetition.com 1337):

> help
Alien's shell
Type program names and arguments, and hit enter.
The following are built in:
  cd
  help
  exit
Use the man command for information on other programs.
>

So clearly this is some kind of shell, let’s try doing an ls:

ls -l
total 8
----------    1 1338     1338            33 Jul  4 12:07 ORME.flag
-r--------    1 1338     1338            28 Jul  4 12:07 README.flag

Now we can see that we can at least read README.flag, trying to do that we find:

> cat README.flag
error: No such file or directory

So we find we can’t use cat or any other kind of usual command to output the flag. How do we know this? Let’s try listing all the commands avaialabe in the /usr/bin/. We quickly find that there are no commands such as cat, less etc.

Okay so let’s try enumerating all the interesting directories where we can find some obscure program to output our flag:

> ls -l /bin /sbin /usr/bin /usr/sbin                                         
/bin:
total 800
lrwxrwxrwx    1 65534    65534           12 May  9 20:49 arch -> /bin/busybox
-rwxr-xr-x    1 65534    65534       796240 Jan 24 07:45 busybox
lrwxrwxrwx    1 65534    65534           12 May  9 20:49 chgrp -> /bin/busybox
lrwxrwxrwx    1 65534    65534           12 May  9 20:49 chown -> /bin/busybox
lrwxrwxrwx    1 65534    65534           12 May  9 20:49 conspy -> /bin/busybox
..............
/sbin:
total 228
lrwxrwxrwx    1 65534    65534           12 May  9 20:49 acpid -> /bin/busybox
lrwxrwxrwx    1 65534    65534           12 May  9 20:49 adjtimex -> /bin/busybox
..............
/usr/bin:
total 1984
lrwxrwxrwx    1 65534    65534           12 May  9 20:49 [ -> /bin/busybox
lrwxrwxrwx    1 65534    65534           12 May  9 20:49 [[ -> /bin/busybox
..............
-rwxr-xr-x    1 65534    65534        25216 Mar 19 09:56 iconv
..............
-rwxr-xr-x    1 65534    65534        83744 Nov 15  2018 scanelf
..............
/usr/sbin:
total 16
lrwxrwxrwx    1 65534    65534           12 May  9 20:49 addgroup -> /bin/busybox
lrwxrwxrwx    1 65534    65534           12 May  9 20:49 adduser -> /bin/busybox
..............
> 

Looking through the list I happen to come across two commands I have not seen or noticed before! The commands are: shuf and iconv. We look up the man pages for them and find that both of them write file contents to stdout! Exactly what we want to do:

> shuf README.flag
CTF{4ll_D474_5h4ll_B3_Fr33}

> iconv /challenge/README.flag
CTF{4ll_D474_5h4ll_B3_Fr33}

Here’s our flag!

FriendSpaceBookPlusAllAccessRedPremium.com - Reversing

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