CVE-2023-51766

Exp

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.

Published: 2023-12-24 Last update: 2025-11-04 Assigner: [email protected] Source: [email protected]
NVD Status：Modified，CVE State: published
View at NVD, CVE.org, EUVD

CVE-2023-51766 is rated Exploit Available (56.8/100): CVSS Medium severity, with medium exploitation likelihood (EPSS 1.07%). 1 public exploit reference(s) are indexed (Exploit-DB). Public exploits are available—assess exposure, apply mitigations, and prioritize patching.

Risk is dynamic; we continuously reassess and refresh what is shown on this page as upstream context changes.

Public exploit references (Exploit-DB) for CVE-2023-51766

EDB-ID Source Kind Published Link
— nvd_ref exploit_tag Exploit-DB ↗

Exploit prediction scoring system (EPSS) score for CVE-2023-51766

EPSS lead: Daily EPSS estimates relative likelihood of exploitation; percentile ranks this CVE among scored vulnerabilities (higher = more severe relative rank).

# Date Old EPSS score New EPSS score Delta (New - Old)
1 2026-06-15 1.64% 1.07% -0.57%
2 2025-12-28 0.97% 1.64% +0.68%
3 2025-12-27 — 0.97% —

Full EPSS history (28 records total)

Common vulnerability scoring system (CVSS) metrics for CVE-2023-51766

CVSS metrics for this CVE.

Base score Version Severity Vector Exploitability Impact Score source
5.3 3.1 MEDIUM
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N Click to expand
Attack vector (AV:N)
Could be attacked over the internet or any normal routed network—not just someone sitting at the machine.
Attack complexity (AC:L)
Once they can reach the bug, pulling it off is straightforward—no weird race conditions or rare setup.
Privileges required (PR:N)
No account or special rights needed—anonymous or random user is enough.
User interaction (UI:N)
Nobody has to click “OK” or open a trap file; it can work without a victim helping.
Scope (S:U)
Damage stays in the same “trust bubble” as the broken component—no big spill into unrelated systems.
Confidentiality (C:N)
Doesn’t really leak secrets in a meaningful way.
Integrity (I:L)
Attackers could change some data, but it’s limited—not everything goes.
Availability (A:N)
Service keeps running; no real outage angle.
 3.9 1.4 [email protected]

Weakness enumeration for CVE-2023-51766

OS Trackers for CVE-2023-51766

vendor priority summary link
alpine — CVE-2023-51766: 1 source package rows (exim); 35 state rows across 5 repos (3.19-community, 3.20-community, 3.21-community, 3.22-community, edge-community); fixed 5, open 30. https://security.alpinelinux.org/vuln/CVE-2023-51766
debian not yet assigned CVE-2023-51766 not yet assigned priority: Debian including 1 source packages (exim4), 5 status rows across 5 suites (bookworm, bullseye, forky, sid, trixie): resolved 5. https://security-tracker.debian.org/tracker/CVE-2023-51766
gentoo high CVE-2023-51766: 1 GLSA(s) (202402-18), 1 atom(s) (mail-mta/exim); latest impact high. https://bugs.gentoo.org/buglist.cgi?quicksearch=CVE-2023-51766
ubuntu medium CVE-2023-51766 medium priority: Ubuntu including 1 source packages (exim4), 12 status rows across 12 suites (bionic, focal, jammy, lunar, mantic, noble, oracular, plucky, questing, trusty, upstream, xenial): released 10, ignored 1, needs-triage 1. https://ubuntu.com/security/CVE-2023-51766

Affected software / configurations for CVE-2023-51766

Vendor Product Version Raw CPE
exim exim < 4.97.1 cpe:2.3:a:exim:exim:*:*:*:*:*:*:*:*
fedoraproject extra_packages_for_enterprise_linux 7.0 cpe:2.3:a:fedoraproject:extra_packages_for_enterprise_linux:7.0:*:*:*:*:*:*:*
fedoraproject extra_packages_for_enterprise_linux 8.0 cpe:2.3:a:fedoraproject:extra_packages_for_enterprise_linux:8.0:*:*:*:*:*:*:*
fedoraproject extra_packages_for_enterprise_linux 9.0 cpe:2.3:a:fedoraproject:extra_packages_for_enterprise_linux:9.0:*:*:*:*:*:*:*
fedoraproject fedora 38 cpe:2.3:o:fedoraproject:fedora:38:*:*:*:*:*:*:*
fedoraproject fedora 39 cpe:2.3:o:fedoraproject:fedora:39:*:*:*:*:*:*:*
debian debian_linux 10.0 cpe:2.3:o:debian:debian_linux:10.0:*:*:*:*:*:*:*

References for CVE-2023-51766

URL Tags
http://www.openwall.com/lists/oss-security/2023/12/24/1 Mailing List Third Party Advisory
http://www.openwall.com/lists/oss-security/2023/12/25/1 Mailing List Third Party Advisory
http://www.openwall.com/lists/oss-security/2023/12/29/2 Mailing List Mitigation Third Party Advisory
http://www.openwall.com/lists/oss-security/2024/01/01/1 Mailing List Third Party Advisory
http://www.openwall.com/lists/oss-security/2024/01/01/2 Mailing List Third Party Advisory
http://www.openwall.com/lists/oss-security/2024/01/01/3 Mailing List Third Party Advisory
https://bugs.exim.org/show_bug.cgi?id=3063 Issue Tracking Vendor Advisory
https://bugzilla.redhat.com/show_bug.cgi?id=2255852 Issue Tracking Third 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 Mitigation Vendor Advisory
https://lists.debian.org/debian-lts-announce/2024/01/msg00002.html Mailing List Third Party Advisory
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/ORN7OKEQPPBKUHYRQ6LR5PSNBQVDHAWB/ Mailing List Third Party Advisory
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/QPDWHJPABVJCXDSNELSSVTIVAJU2MDUQ/ Mailing List Third Party Advisory
https://lwn.net/Articles/956533/ Third Party Advisory
https://sec-consult.com/blog/detail/smtp-smuggling-spoofing-e-mails-worldwide/ Technical Description Third Party Advisory
https://www.openwall.com/lists/oss-security/2023/12/23/2 Issue Tracking Mailing List Third Party Advisory
https://www.youtube.com/watch?v=V8KPV96g1To Exploit
https://lists.fedoraproject.org/archives/list/[email protected]/message/ORN7OKEQPPBKUHYRQ6LR5PSNBQVDHAWB/
https://lists.fedoraproject.org/archives/list/[email protected]/message/QPDWHJPABVJCXDSNELSSVTIVAJU2MDUQ/
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