pam_shells.so 
      pam_shells is a PAM module that only allows access to the
      system if the user's shell is listed in /etc/shells.
    
      If this file does not exist, entries are taken from files
      %vendordir%/shells,
      %vendordir%/shells.d/* and
      /etc/shells.d/* in that order.
    
      It also checks if needed files (e.g. /etc/shells) are plain
      files and not world writable.
    
Access to the system was denied.
            The user's login shell was listed as valid shell in
            /etc/shells.
          
The user does not exist or the user's login shell could not be determined.
The module was not able to get the name of the user.