CVE-2023-51766

Severity: medium (CVSS 5.3 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 1.1% probability of exploitation in the next 30 days
Public exploit or detection template: yes
Published: 2023-12-24 · updated 2026-06-17
Weakness: CWE-345

Description

Exim before 4.97.1 allows SMTP smuggling in certain PIPELINING/CHUNKING configurations. 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 Exim supports <LF>.<CR><LF> but some other popular e-mail servers do not.

Affected products

Debian Linux: 10.0
Exim: before 4.97.1 — fixed in 4.97.1
Fedoraproject Extra Packages For Enterprise Linux: 7.0, 8.0, 9.0
Fedoraproject Fedora: 38, 39

How to fix

Upgrade to a fixed version: Exim 4.97.1. 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/2023/12/29/2
http://www.openwall.com/lists/oss-security/2024/01/01/1
http://www.openwall.com/lists/oss-security/2024/01/01/2
http://www.openwall.com/lists/oss-security/2024/01/01/3
https://bugs.exim.org/show_bug.cgi?id=3063
https://bugzilla.redhat.com/show_bug.cgi?id=2255852
https://exim.org/static/doc/security/CVE-2023-51766.txt
https://fahrplan.events.ccc.de/congress/2023/fahrplan/events/11782.html
https://git.exim.org/exim.git/commit/5bb786d5ad568a88d50d15452aacc8404047e5ca
https://git.exim.org/exim.git/commit/cf1376206284f2a4f11e32d931d4aade34c206c5

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