Postfix through 3.8.4 allows SMTP smuggling unless configured with smtpd_data_restrictions=reject_unauth_pipelining (or certain other options that exist in recent versions). Remote attackers can use a published exploitation technique to inject e-mail messages that appear to originate from the Postfix server, allowing bypass of an SPF protection mechanism. This occurs because Postfix supports . but some other popular e-mail servers do not. To prevent attack variants (by always disallowing without ), a different solution is required: the smtpd_forbid_bare_newline=yes option with a Postfix minimum version of 3.5.23, 3.6.13, 3.7.9, 3.8.4, or 3.9.

References

• https://nvd.nist.gov/vuln/detail/CVE-2023-51764
• https://sec-consult.com/blog/detail/smtp-smuggling-spoofing-e-mails-worldwide/
• https://www.postfix.org/smtp-smuggling.html
• http://www.openwall.com/lists/oss-security/2023/12/24/1
• https://access.redhat.com/security/cve/CVE-2023-51764
• https://bugzilla.redhat.com/show_bug.cgi?id=2255563
• https://fahrplan.events.ccc.de/congress/2023/fahrplan/events/11782.html
• http://www.openwall.com/lists/oss-security/2023/12/25/1
• https://github.com/duy-31/CVE-2023-51764
• https://github.com/eeenvik1/CVE-2023-51764
• https://www.youtube.com/watch?v=V8KPV96g1To
• https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/QRLF5SOS7TP5N7FQSEK2NFNB44ISVTZC/
• https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/JQ5WXFCW2N6G2PH3JXDTYW5PH5EBQEGO/
• https://lwn.net/Articles/956533/
• https://lists.debian.org/debian-lts-announce/2024/01/msg00020.html
• https://www.openwall.com/lists/oss-security/2024/01/22/1
• https://www.postfix.org/announcements/postfix-3.8.5.html