Information
The shadow group allows system programs which require access the ability to read the /etc/shadow file. No users should be assigned to the shadow group.
Any users assigned to the shadow group would be granted read access to the /etc/shadow file. If attackers can gain read access to the /etc/shadow file, they can easily run a password cracking program against the hashed passwords to break them. Other security information that is stored in the /etc/shadow file (such as expiration) could also be useful to subvert additional user accounts.
Solution
Run the following command to remove all users from the shadow group
# sed -ri 's/(^shadow:[^:]*:[^:]*:)([^:]+$)/1/' /etc/group
Change the primary group of any users with shadow as their primary group.
# usermod -g <primary group> <user>