CVE-2023-51764: Fedoraproject Fedora

Medium severity, CVSS 5.3. EPSS: 2.6% chance of exploitation in the next 30 days.

Postfix through 3.8.5 allows SMTP smuggling unless configured with smtpd_data_restrictions=reject_unauth_pipelining and smtpd_discard_ehlo_keywords=chunking (or certain other options that exist in recent versions). Remote attackers can use a published exploitation technique to inject e-mail messages with a spoofed MAIL FROM address, allowing bypass of an SPF protection mechanism. This occurs because Postfix supports <LF>.<CR><LF> but some other popular e-mail servers do not. To prevent attack variants (by always disallowing <LF> without <CR>), a different solution is required, such as 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.

Affected products

• Fedoraproject Fedora: version 38 only; version 39 only
• Postfix Postfix: before 3.5.23 (fixed in 3.5.23); from 3.6.0, before 3.6.13 (fixed in 3.6.13); from 3.7.0, before 3.7.9 (fixed in 3.7.9); from 3.8.0, before 3.8.4 (fixed in 3.8.4)
• Red Hat Enterprise Linux: version 8.0 only; version 9.0 only

Published 2023-12-24. Last modified 2026-10-08.