Unable to secure WordPress instance: Permissions for files and directories still marked as Not Secure

Symptoms

• When clicking on Secure button at Plesk > Domains > example.com > Check Security nothing happens: there is still orange exclamation mark next to Permissions for files and directories;
• There are several files or directories inside a domain document root directory that do not belong to psacln group:

  # find /var/www/vhosts/example.com/httpdocs/* ! -group psacln
  drwxr-xr-x. 9 sysuser apache 4096 Aug 11 2016 wp-admin
  -rw-r--r--. 1 sysuser apache 4035 Sep 18 2015 wp-trackback.php

Cause

Incorrect ownership: all files and directories under a domain document root directory should belong to psacln group.

Resolution

1. Connect to the server using SSH.
2. Modify the ownership for all files and directories inside domain's document root:

   # chown -R :psacln /var/www/vhosts/example.com/httpdocs/*

   or, if the subdomain is affected:

   # chown -R :psacln /var/www/vhosts/example.com/subdom.example.com/*

3. Set group ownership for httpdocs directory itself to psaserv :

   # chown :psaserv /var/www/vhosts/example.com/httpdocs

   or, if the subdomain is affected:

   # chown :psaserv /var/www/vhosts/example.com/subdom.example.com/