Information
An account with an empty password field means that anybody may log in as that user without providing a password at all (assuming that the value PASSREQ=NO is set in /etc/default/login).
All accounts must have passwords, be configured as "Non-login," or be locked.
Solution
Use the passwd -l command to lock accounts that are not permitted to execute commands. Use the passwd -N command to set accounts to be non-login.