[An on-line version of this announcement will be available at https://www.postfix.org/announcements/postfix-3.8.4.html]
Fixed with Postfix 3.8.4:
Security: this release adds support to defend against an email spoofing attack (SMTP smuggling) on recipients at a Postfix server. For background, see https://www.postfix.org/smtp-smuggling.html.
Sites concerned about SMTP smuggling attacks should enable this feature on Internet-facing Postfix servers. For compatibility with non-standard clients, Postfix by default excludes clients in mynetworks from this countermeasure.
The recommended settings are:
# Optionally disconnect remote SMTP clients that send bare newlines, # but allow local clients with non-standard SMTP implementations # such as netcat, fax machines, or load balancer health checks. # smtpd_forbid_bare_newline = yes smtpd_forbid_bare_newline_exclusions = $mynetworks
The smtpd_forbid_bare_newline feature is disabled by default.
You can find the updated Postfix source code at the mirrors listed at https://www.postfix.org/.