# CVE-2023-51766: Security feature bypass in n/a

> Remote attackers can bypass SPF protection and spoof sender addresses in Exim mail servers using SMTP smuggling techniques.

- **Severity:** Medium 5.3 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 24 Dec 2023
- **Updated:** 4 Nov 2025
- **Also known as:** CLEANSTART-2025-LV68873, CLSA-2024-1709562273, CLSA-2024-1709563071, DEBIAN-CVE-2023-51766, DLA-3708-1, DSA-5597-1, GHSA-w6ww-869j-cgm6, OESA-2024-1928, UBUNTU-CVE-2023-51766, USN-6611-1, openSUSE-SU-2024:13543-1

## What is CVE-2023-51766?

CVE-2023-51766 is a medium-severity vulnerability in Debian Debian Linux, published on 24 Dec 2023. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official 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.

Weakness: CWE-345 (Insufficient Verification of Data Authenticity)

## Which versions are affected by CVE-2023-51766?

It affects 4 products.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `exim` | openSUSE:Tumbleweed | All versions | 4.97.1-1.1 |

Products: Debian Debian Linux, Exim Exim, Fedoraproject Extra Packages for Enterprise Linux, Fedoraproject Fedora.

## How do I fix CVE-2023-51766?

Upgrade to a fixed version.

## Is CVE-2023-51766 being exploited?

There is no known exploitation of CVE-2023-51766 in the wild and no public exploit code.

## Other vulnerabilities in Debian Debian Linux

- [CVE-2015-3306](https://insights.sbomify.com/CVE-2015-3306/): ProFTPD Improper Access Control (critical 10.0, exploited in the wild)
- [CVE-2025-49113](https://insights.sbomify.com/CVE-2025-49113/): RoundCube Webmail Deserialization of Untrusted Data (critical 9.9, exploited in the wild)
- [CVE-2026-24061](https://insights.sbomify.com/CVE-2026-24061/): GNU InetUtils Argument Injection (critical 9.8, exploited in the wild)
- [CVE-2025-39682](https://insights.sbomify.com/CVE-2025-39682/): Linux Kernel Improper Check for Unusual or Exceptional Conditions (critical 9.8, exploited in the wild)
- [CVE-2024-9680](https://insights.sbomify.com/CVE-2024-9680/): Mozilla Firefox Use-After-Free (critical 9.8, exploited in the wild)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2023-51766 and fix it if it is affected.

What it is: CVE-2023-51766 is a medium-severity vulnerability in Debian Debian Linux, published on 24 Dec 2023. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2023-51766 in the wild and no public exploit code.

Affected product: Debian Debian Linux. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2023-51766/ (Markdown: https://insights.sbomify.com/CVE-2023-51766/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2023-51766
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2023-51766
- OSV: https://osv.dev/vulnerability/CVE-2023-51766

---
From sbomify Insights: https://insights.sbomify.com/CVE-2023-51766/
Summaries are generated and may contain mistakes; the official description above is the record.