What’s Vulnerable › Vulnerabilities

CVE-2023-51764

• Severity: medium (CVSS 5.3 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 2.6% probability of exploitation in the next 30 days
• Public exploit or detection template: yes
• Published: 2023-12-24 · updated 2026-10-08
• Weakness: CWE-345

Description

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: 38, 39
• Postfix: before 3.5.23; from 3.6.0 before 3.6.13; from 3.7.0 before 3.7.9; from 3.8.0 before 3.8.4 — fixed in 3.5.23, 3.6.13, 3.7.9, 3.8.4
• Red Hat Enterprise Linux: 8.0, 9.0

How to fix

Upgrade to a fixed version: Postfix 3.5.23, 3.6.13, 3.7.9, 3.8.4. Check the vendor advisory for mitigations.

References

• http://www.openwall.com/lists/oss-security/2023/12/24/1
• http://www.openwall.com/lists/oss-security/2023/12/25/1
• http://www.openwall.com/lists/oss-security/2024/05/09/3
• 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
• https://github.com/duy-31/CVE-2023-51764
• https://github.com/eeenvik1/CVE-2023-51764
• https://lists.debian.org/debian-lts-announce/2024/01/msg00020.html
• https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/JQ5WXFCW2N6G2PH3JXDTYW5PH5EBQEGO/
• https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/QRLF5SOS7TP5N7FQSEK2NFNB44ISVTZC/
• https://lwn.net/Articles/956533/

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