I have a user user1 that is not in the sudoers file (nor belonging to groups in sudoer) but can still use sudo. How is it possible ?
user1~$ groups
user1 www-data
My sudoer conf
root ALL=(ALL) ALL
%sudo ALL=(ALL) ALL
#includedir /etc/sudoers.d
user1~$ sudo -l
User user1 may run the following commands on this host:
(ALL) ALL