How to authenticate user without shell access to ftp server vsftp without adding nologin in /etc/shells on RHEL 7.6.

Issue

• With a user configured with nologin:

  # grep user1 /etc/passwd
  user1:x:1000:1000::/home/user1:/sbin/nologin
  

• The user is not authorised to authenticate through ftp:

  $ ftp 192.168.xxx.xxx 
  Connected to 192.168.xxx.xxx (192.168.xxx.xxx).
  220 FTP server - Authorized Use Only!
  Name (192.168.xxx.xxx:user): user1
  331 Please specify the password.
  Password:
  530 Login incorrect.
  Login failed.
  ftp> 
  

• One solution is to re-add nologin in the list of shells but this will go against the CVE-2018-1113 described in the Root cause section, thus do not do it as there is a security risk:

  # grep login /etc/shells
  /bin/nologin