CVE-2023-51766 — Exim before 4

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

01What is this vulnerability?

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.

02Affected products

Vendor	Product	Versions
n	a / n/a	n/a

03Active exploitation status

Not currently listed on the CISA Known Exploited Vulnerabilities (KEV) catalog. EPSS is the best forward-looking signal — see the EPSS row above.

04Recommended remediation

• Patch to a fixed version listed in the vendor advisory (see references below).
• Mitigate with WAF rules, network egress filters, or feature flags where the patch is not yet available.
• Hunt historical logs for exploitation indicators — see Detection signatures below.

05Technical details

For the full vendor write-up, exploit chains, and reference implementations, see the references list in section 09.

06Detection signatures

Ask SARA in chat to draft a starting detection for this CVE in your stack of choice.

07Related CVEs

No directly-cited follow-up CVEs in the KB record for this advisory. The references list in section 09 carries the vendor cross-references.

08Timeline

• Published: 2023-12-24T00:00:00.000Z
• Last modified: 2025-11-04T18:21:35.998Z

09References

• sec-consult.com — https://sec-consult.com/blog/detail/smtp-smuggling-spoofing-e-mails-worldwide/
• exim.org — https://exim.org/static/doc/security/CVE-2023-51766.txt
• bugs.exim.org — https://bugs.exim.org/show_bug.cgi?id=3063
• www.openwall.com — https://www.openwall.com/lists/oss-security/2023/12/23/2
• www.openwall.com — http://www.openwall.com/lists/oss-security/2023/12/24/1
• www.openwall.com — http://www.openwall.com/lists/oss-security/2023/12/25/1
• git.exim.org — https://git.exim.org/exim.git/commit/5bb786d5ad568a88d50d15452aacc8404047e5ca
• git.exim.org — https://git.exim.org/exim.git/commit/cf1376206284f2a4f11e32d931d4aade34c206c5
• bugzilla.redhat.com — https://bugzilla.redhat.com/show_bug.cgi?id=2255852
• fahrplan.events.ccc.de — https://fahrplan.events.ccc.de/congress/2023/fahrplan/events/11782.html
• www.openwall.com — http://www.openwall.com/lists/oss-security/2023/12/29/2
• github.com — https://github.com/Exim/exim/blob/master/doc/doc-txt/cve-2023-51766
• www.openwall.com — http://www.openwall.com/lists/oss-security/2024/01/01/1
• www.openwall.com — http://www.openwall.com/lists/oss-security/2024/01/01/2
• www.openwall.com — http://www.openwall.com/lists/oss-security/2024/01/01/3
• www.youtube.com — https://www.youtube.com/watch?v=V8KPV96g1To
• lists.debian.org — https://lists.debian.org/debian-lts-announce/2024/01/msg00002.html
• lists.fedoraproject.org — https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedorapro…
• lists.fedoraproject.org — https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedorapro…
• lwn.net — https://lwn.net/Articles/956533/