SideDoor - Debian/Ubuntu Backdoor Using A Reverse SSH Tunnel

Wednesday, February 10, 2016

sidedoor maintains a reverse tunnel to provide a backdoor. sidedoor can be used to remotely control a device behind a NAT.

sidedoor is packaged for Debian-based systems with systemd or upstart. It has been used on Debian 8 (jessie) and Ubuntu 14.04 LTS (trusty).

The sidedoor user has full root access configured in /etc/sudoers.d.

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

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 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  

  • 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 ).
    • 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.

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