From gbhackers.com
Critical Remote command execution vulnerability that affected Exim Email Server versions 4.87 to 4.91 let a local attacker or a remote attacker(with limited boundary) can execute an arbitrary command and exploit the server.
Exim is a mail transfer agent which is used on Unix-like operating systems for sending, receiving and routing the email messages. also, it is free software distributed under the terms of the General Public License (GNU)
Both local and remote attacker could exploit this vulnerability, but there are certain non-default configurations restriction take place for the remote attacker in order to execute the arbitrary commands.