SideDoor – Debian/Ubuntu Backdoor Using A Reverse SSH Tunnel

If sidedoor is in your package repositories, simply install it, e.g., sudo apt-get install sidedoor . Otherwise, you will need to build a Debian package and install it. First, install build dependencies.

sudo apt-get install debhelper dh-systemd  

Then, from the directory containing this README file, build and install a package.

rm -f ../sidedoor*.deb # remove old package build
dpkg-buildpackage -us -uc -b
sudo dpkg -i ../sidedoor*.deb

Configuration
The remote server and tunnel port are configured in /etc/default/sidedoor . SSH configuration files are located in the /etc/sidedoor directory. ~sidedoor/.ssh is a symlink to /etc/sidedoor .

• Configure REMOTE_SERVER and TUNNEL_PORT in /etc/default/sidedoor .
• Create SSH configuration files under /etc/sidedoor .
  • authorized_keys : SSH public key(s) to control access to the local sidedoor user.
  • id_rsa : SSH private key to access the remote server. Can be generated with sudo ssh-keygen -t rsa -f /etc/sidedoor/id_rsa (press enter when prompted for passphrase to leave empty). Needs read permission by the sidedoor user or group, e.g., sudo chown root:sidedoor /etc/sidedoor/id_rsa and sudo chmod 640 /etc/sidedoor/id_rsa . The corresponding public key id_rsa.pub will need to be included in the remote user’s ~/.ssh/authorized_keys file.
  • known_hosts : SSH host key of the remote server.
  • config (optional): Additional SSH config, see man ssh_config .

Restart the sidedoor service to apply changes.

sudo service sidedoor restart  

Recommendations

• Lock down the local SSH server by editing /etc/ssh/sshd_config .
  • Disable password authentication ( ChallengeResponseAuthentication no and PasswordAuthentication no ).
  • Limit daemon to only listen on localhost. ( ListenAddress ::1 and ListenAddress 127.0.0.1 ).
  • To apply changes, restart or reload sshd, e.g., sudo service ssh reload .
• Modify the ssh_client_config_example file and include it in a client’s ~/.ssh/config file to easily access the tunneled backdoor with ssh , scp , rsync , etc.