Exim through 4.97 allows SMTP smuggling in certain configurations. Remote attackers can use a published exploitation technique to inject e-mail messages that appear to originate from the Exim server, allowing bypass of an SPF protection mechanism. This occurs because Exim supports . but some other popular e-mail servers do not.

References

• https://nvd.nist.gov/vuln/detail/CVE-2023-51766
• https://bugs.exim.org/show_bug.cgi?id=3063
• https://exim.org/static/doc/security/CVE-2023-51766.txt
• https://sec-consult.com/blog/detail/smtp-smuggling-spoofing-e-mails-worldwide/
• https://www.openwall.com/lists/oss-security/2023/12/23/2
• http://www.openwall.com/lists/oss-security/2023/12/24/1
• https://bugzilla.redhat.com/show_bug.cgi?id=2255852
• 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
• http://www.openwall.com/lists/oss-security/2023/12/25/1
• http://www.openwall.com/lists/oss-security/2023/12/29/2
• https://github.com/Exim/exim/blob/master/doc/doc-txt/cve-2023-51766
• 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://lists.debian.org/debian-lts-announce/2024/01/msg00002.html
• https://www.youtube.com/watch?v=V8KPV96g1To
• https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/ORN7OKEQPPBKUHYRQ6LR5PSNBQVDHAWB/
• https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/QPDWHJPABVJCXDSNELSSVTIVAJU2MDUQ/
• https://lwn.net/Articles/956533/