The receive_msg function in receive.c in the SMTP daemon in Exim 4.88 and 4.89 allows remote attackers to execute arbitrary code or cause a denial of service (use-after-free) via vectors involving BDAT commands.
if you are running Exim 4.88 or newer, then in the main section of your Exim configuration, set:
chunking_advertise_hosts =
This disables advertising the ESMTP CHUNKING extension, making the BDAT verb unavailable and avoids letting an attacker apply the logic.