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Vulnerability record · CVE-2023-51766 · published 24 December 2023

CVE-2023-51766: Exim insufficient verification of data authenticity vulnerability

Exim · Exim

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.

5.3 CVSS 3.1 Medium EPSS 1.1% · top 35.6% CWE-345 · Insufficient verification of data authenticity
5.3CVSS 3.1 base score
1.1%EPSS exploitation probability, 30 days
NoNot in CISA KEV
4Affected product versions listed by NVD
42References, 2 tagged exploit
17 Jun 2026Last modified by NVD

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.

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N

Affected products

4 vulnerable configurations from NVD's CPE data, grouped by vendor and product.

References

LinkTags
http://www.openwall.com/lists/oss-security/2023/12/24/1 Mailing ListThird Party Advisory
http://www.openwall.com/lists/oss-security/2023/12/25/1 Mailing ListThird Party Advisory
http://www.openwall.com/lists/oss-security/2023/12/29/2 Mailing ListMitigationThird Party Advisory
http://www.openwall.com/lists/oss-security/2024/01/01/1 Mailing ListThird Party Advisory
http://www.openwall.com/lists/oss-security/2024/01/01/2 Mailing ListThird Party Advisory
http://www.openwall.com/lists/oss-security/2024/01/01/3 Mailing ListThird Party Advisory
https://bugs.exim.org/show_bug.cgi?id=3063 Issue TrackingVendor Advisory
https://bugzilla.redhat.com/show_bug.cgi?id=2255852 Issue TrackingThird Party Advisory
https://exim.org/static/doc/security/CVE-2023-51766.txt Broken Link
https://fahrplan.events.ccc.de/congress/2023/fahrplan/events/11782.html Technical Description
https://git.exim.org/exim.git/commit/5bb786d5ad568a88d50d15452aacc8404047e5ca Patch
https://git.exim.org/exim.git/commit/cf1376206284f2a4f11e32d931d4aade34c206c5 Patch
https://github.com/Exim/exim/blob/master/doc/doc-txt/cve-2023-51766 MitigationVendor Advisory
https://lists.debian.org/debian-lts-announce/2024/01/msg00002.html Mailing ListThird Party Advisory
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/ORN7OKEQPPBKUHYRQ6LR5PS Mailing ListThird Party Advisory
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/QPDWHJPABVJCXDSNELSSVTI Mailing ListThird Party Advisory
https://lwn.net/Articles/956533/ Third Party Advisory
https://sec-consult.com/blog/detail/smtp-smuggling-spoofing-e-mails-worldwide/ Technical DescriptionThird Party Advisory
https://www.openwall.com/lists/oss-security/2023/12/23/2 Issue TrackingMailing ListThird Party Advisory
https://www.youtube.com/watch?v=V8KPV96g1To Exploit
http://www.openwall.com/lists/oss-security/2023/12/24/1 Mailing ListThird Party Advisory
http://www.openwall.com/lists/oss-security/2023/12/25/1 Mailing ListThird Party Advisory
http://www.openwall.com/lists/oss-security/2023/12/29/2 Mailing ListMitigationThird Party Advisory
http://www.openwall.com/lists/oss-security/2024/01/01/1 Mailing ListThird Party Advisory
http://www.openwall.com/lists/oss-security/2024/01/01/2 Mailing ListThird Party Advisory
http://www.openwall.com/lists/oss-security/2024/01/01/3 Mailing ListThird Party Advisory
https://bugs.exim.org/show_bug.cgi?id=3063 Issue TrackingVendor Advisory
https://bugzilla.redhat.com/show_bug.cgi?id=2255852 Issue TrackingThird Party Advisory
https://exim.org/static/doc/security/CVE-2023-51766.txt Broken Link
https://fahrplan.events.ccc.de/congress/2023/fahrplan/events/11782.html Technical Description
https://git.exim.org/exim.git/commit/5bb786d5ad568a88d50d15452aacc8404047e5ca Patch
https://git.exim.org/exim.git/commit/cf1376206284f2a4f11e32d931d4aade34c206c5 Patch
https://github.com/Exim/exim/blob/master/doc/doc-txt/cve-2023-51766 MitigationVendor Advisory
https://lists.debian.org/debian-lts-announce/2024/01/msg00002.html Mailing ListThird Party Advisory
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/ORN7OKEQPPBKUHYRQ6LR5PS Mailing ListThird Party Advisory
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/QPDWHJPABVJCXDSNELSSVTI Mailing ListThird Party Advisory
https://lists.fedoraproject.org/archives/list/[email protected]/message/ORN7OKEQPPBKUHYRQ6LR5PSNB
https://lists.fedoraproject.org/archives/list/[email protected]/message/QPDWHJPABVJCXDSNELSSVTIVA
https://lwn.net/Articles/956533/ Third Party Advisory
https://sec-consult.com/blog/detail/smtp-smuggling-spoofing-e-mails-worldwide/ Technical DescriptionThird Party Advisory

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Source: NIST National Vulnerability Database (record CVE-2023-51766), CISA KEV, FIRST EPSS (scores of 2026-09-30). This page is refreshed as NVD updates the record.