MariaDB [(none)]> use mysql;
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A

Database changed
MariaDB [mysql]> UPDATE user SET plugin='mysql_native_password' WHERE User='root';
Query OK, 0 rows affected (0.017 sec)
Rows matched: 1  Changed: 0  Warnings: 0

MariaDB [mysql]> flush privileges;
Query OK, 0 rows affected (0.000 sec)

MariaDB [mysql]> exit
Bye

# 권한 설정 변경
root@secu-VirtualBox:~# cd /var/www/html/board
root@secu-VirtualBox:/var/www/html/board# chown www-data:www-data -R /var/www/html/board/