sshcatch

A quick-deploy SSH server for tunneling (local/remote/dynamic) and simple SCP / SFTP transfers - it never opens a shell.

By default all features are disabled: connections are logged and closed. Turn on only what you need with the flags described below. Handy on an engagement when you want a controlled SSH endpoint (a tunnel relay or a file drop) without setting up a full sshd.

Built on asyncssh.

This is a pentesting tool. Only point it at systems and networks you are authorized to test.

Why this tool exists

• During engagements and CTFs I love to use 'simple' tools on my host that just work

  • http-server -> python3 -m http.server
  • smb-server -> impacket-smbserver
  • ssh-server -> ??? (now sshcatch)

• sshd can be used, but:

  • configuring it through sshd_configs is a pain
  • multiple use-cases require different configs (tunnel direction? sftp direction? different ports?)
  • logins are controlled by the OS so a user must be created (and secured)
  • I want to restrict the shell so my host is safe

• My solution: sshcatch

  • Simply configure through clear flags and arguments on the commandline
    • restrictive defaults, every feature must be enabled consciously
  • Never allow shells (or commands)
  • Forward/Reverse tunnels can be individually activated
  • SCP/SFTP file uploads and downloads can be individually activated
    • restrictive upload handling to prevent overwriting
    • symlinks denied

Install

With pipx (recommended, installs into an isolated environment and puts sshcatch on your PATH):

pipx install sshcatch

With pip:

pip install sshcatch

From source:

git clone https://github.com/LorenzMap/sshcatch
cd sshcatch
pipx install .          # or: pip install .

Needs Python 3.10+. Three host keys (ed25519, RSA, ECDSA) are auto-generated into a single file in the working directory on first run (or point --host-key at your own file).

How it works

Without any flags sshcatch is in log-only mode: It accepts the connection, records the client version, username, offered passwords and public keys, then closes. Nothing else is enabled until you ask for it.

Adjust the amount of logging using -vv and -q. Or put everything into a logfile using -o.

Tunnels

Because no shell is ever created, tunnel clients must pass -N (e.g. ssh -NL ...) or they get disconnected instantly.

Turning tunneling on with --open-auth means anyone who connects can pivot through your host!

Only plain TCP forwards are ever available. UNIX-domain-socket forwards (ssh -L /sock:... / -R /sock:...) and TUN/TAP tunnels (ssh -w) are always denied, even with --forward / --reverse set.

SCP / SFTP

Symlinks are handled very restrictively: On upload they create a placeholder file that contains the original target. On download they are outright denied.

The host key, authorized_keys, logfile and the sshcatch script itself are protected and hidden when they live inside the SCP directory.

Uploads never overwrite an existing file. The new file gets a numeric suffix (loot.tar -> loot_1.tar). Non-existent parent folders are created.

Renames, deletes and directory removal are denied.

Word of Warning

Only using --version-banner obviously isn't enough deception because the KEXINIT that is transferred cleartext on the wire is a clear tell. This differing HASSH can be easily detected by a sufficiently sophisticated observer.

Also: sshcatch is NOT designed to be a honeypot. Advanced deception, long-term logging and everything else a real honeypot needs are deliberately out of scope. There are other projects that can be used: Cowrie, cyanide-framework and probably a lot more!

Examples

Let one user pull/put files from the current directory via SCP/SFTP:

# Server
sshcatch -u user:pass --scp-download --scp-upload

# Client
scp user@host:secret.txt .
scp -r loot/ user@host:pete/pc/
sftp user@host

Let anyone tunnel through the server (local and dynamic forwards): Be careful with this one!

# Server
sshcatch --open-auth --forward

# Client
ssh -NL 8080:internal:80 user@host      # local forward
ssh -ND 1080 user@host                  # dynamic (SOCKS)

Using single-mode to return something to the first successful authentication by closing the server afterwards, while printing timestamped logs to the console and saving them into a file:

# Server
sshcatch -1 -u arthur:42 --version-banner debian \
         --pre-auth-banner "What is the answer to life the universe and everything" \
         --post-auth-banner "flag{So_Long_and_Thanks_for_All_the_Fish}" \
         -o sshcatch.log -t

My favorite one: Reverse tunnel and SCP uploads for the keys in ./authorized-keys while posing as an Ubuntu SSH server on port 2222:

# Server
sshcatch --reverse --authorized-keys ./authorized-keys --scp-upload --version-banner ubuntu -p 2222

# Client
ssh -NR 9000:localhost:22 user@host -p 2222     # reverse tunnel
scp -P 2222 loot.tar user@host:.                # upload

Options

sshcatch -h prints a short summary with just the flags you need to get going. The full reference below is sshcatch --help:

usage: sshcatch [-h] [--help] [-p PORT] [-b BIND] [-1] [--host-key FILE]
                [--version] [-u USER:PASS] [--open-auth]
                [--authorized-keys FILE] [--forward] [--reverse]
                [--scp-upload] [--scp-download] [--scp-dir DIR]
                [--version-banner STRING] [--pre-auth-banner STRING]
                [--post-auth-banner STRING] [-q | -v] [-o FILE] [-t] [--plain]

sshcatch - a quick-deploy SSH server for tunneling (local/remote/dynamic)
and simple SCP transfers (NEVER opens a shell!).
By default all features are disabled. Use flags to enable features.

options:
  -h                    show a short help message and exit
  --help                show the full help and exit
  -p PORT, --port PORT  listen port (default: 22)
  -b BIND, --bind BIND  bind address (default: all IPv4/v6 interfaces)
  -1, --single          close the listener after first successful auth (and
                        exit when that connection ends)
  --host-key FILE       server host key file, may hold several keys - auto-
                        generated if missing - uses ./sshcatch_host_key by
                        default
  --version             show program's version number and exit

authentication:
  -u USER:PASS, --user USER:PASS
                        allowed user:password (repeatable)
  --open-auth           accept any credentials (open mode)
  --authorized-keys FILE
                        authorized_keys file for key auth (username
                        independent)

tunneling:
  --forward             enable forward tunnels (client: ssh -NL / -ND)
  --reverse             enable reverse tunnels (client: ssh -NR)

SCP / SFTP file transfer:
  --scp-upload          enable file upload (SCP/SFTP write) - files get suffix
                        instead of overwriting
  --scp-download        enable file download (SCP/SFTP read) - symlinks are
                        denied
  --scp-dir DIR         directory for SCP/SFTP (default: cwd) - sensitive
                        sshcatch files (host-key, authorized_keys, logfile)
                        are protected

banners:
  --version-banner STRING
                        sent as 'SSH-2.0-STRING' version banner - only first-
                        glance deception, it can still be identified as
                        asyncssh - presets (case-insensitive): ubuntu, debian,
                        dropbear, windows, macos
  --pre-auth-banner STRING
                        banner shown to every client before login
  --post-auth-banner STRING
                        banner shown only to clients that authenticate
                        successfully

logging:
  -q, --quiet           print nothing on console
  -v, --verbose         print additional information to the console
                        (repeatable)
  -o FILE, --output FILE
                        append the log to FILE (plain with timestamps)
  -t, --timestamps      prefix console lines with a timestamp
  --plain               disable ANSI colors on the console

examples: (also check README on Github)
  sshcatch                                   Log-only (capture creds)
  sshcatch -u user:pass --scp-download       Allow one user to download via SCP/SFTP
  sshcatch --open-auth --forward             Allow ANYONE! to tunnel through this SSH server
  # My favorite one
  #   Allows reverse tunnels and uploads via SCP for the keys in ./authorized_keys
  #   while posing shallowly as an Ubuntu SSH server on port 2222
  sshcatch --reverse --authorized-keys ./authorized-keys --scp-upload --version-banner ubuntu -p 2222

License

MIT