CVE-2023-51765: sendmail through 8.17.2 allows SMTP smuggling in certain configurations

Q: How do I fix CVE-2023-51765?

sendmail: upgrade to 8.18.0.2; freebsd: upgrade to 11.0; enterprise linux: mitigate or apply the listed mitigation. Exact commands for each OS are on this page.

Q: Is CVE-2023-51765 being exploited?

CVE-2023-51765 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

Q: What does CVE-2023-51765 affect?

CVE-2023-51765 affects sendmail, freebsd, enterprise linux. Severity is medium (CVSS 5.3). sendmail through 8.17.2 allows SMTP smuggling in certain 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.

Q: How urgent is CVE-2023-51765?

Priority 50/100. Public exploit reference; Remotely exploitable, no login, no user action; On your watchlist

This week

MEDIUM 5.3REMOTEWATCHLISTUPGRADEUPGRADEMITIGATE

CVE-2023-51765 is a medium-severity flaw (CVSS 5.3) affecting sendmail, freebsd, enterprise linux. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

sendmail through 8.17.2 allows SMTP smuggling in certain 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 sendmail supports <LF>.<CR><LF> but some other popular e-mail servers do not. This is resolved in 8.18 and later versions with 'o' in srv_features.

Why this priority

Public exploit reference
Remotely exploitable, no login, no user action
On your watchlist

A fixed release or distro patch exists.

Risk facts

CVSS score	5.3 (Medium)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	1.1% (top 35%)
CISA exploited list	No
Weakness	CWE-345
Published	2023-12-24
Last modified	2026-06-17

Publisher (CNA): MITRE · Vendor: freebsd · redhat · sendmail

Last updated 2026-06-17 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2023-12-24 06:15 UTCPublished CVE record published
2026-06-17 06:41 UTCUpdated Record last modified

sendmail sendmail UPGRADE

Affected: < 8.18.0.2

Fixed in: 8.18.0.2

Safe to downgrade to: no unaffected older release

1. Check installed version

apt-cache policy sendmail || rpm -q sendmail || sendmail --version

2. UPGRADE to 8.18.0.2 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade sendmail

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade sendmail

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

freebsd freebsd UPGRADE

Affected: < 11.0

Fixed in: 11.0

Safe to downgrade to: no unaffected older release

Check your OS release

cat /etc/os-release; uname -r

Debian / Ubuntu

sudo apt update && sudo apt full-upgrade

RHEL / Amazon Linux / Fedora

sudo dnf upgrade --refresh

Windows

Install the latest cumulative update from Windows Update

If no patch exists yet

OS downgrades are not safe. Apply the vendor mitigation from the advisory and restrict exposure.

redhat enterprise linux MITIGATE

Affected: 8.0 · 9.0

Fixed in: no fixed release known

Safe to downgrade to: a release before 8.0

Check your OS release

cat /etc/os-release; uname -r

Debian / Ubuntu

sudo apt update && sudo apt full-upgrade

RHEL / Amazon Linux / Fedora

sudo dnf upgrade --refresh

Windows

Install the latest cumulative update from Windows Update

If no patch exists yet

OS downgrades are not safe. Apply the vendor mitigation from the advisory and restrict exposure.

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2023-51765?

sendmail: upgrade to 8.18.0.2; freebsd: upgrade to 11.0; enterprise linux: mitigate or apply the listed mitigation. Exact commands for each OS are on this page.

Is CVE-2023-51765 being exploited?

CVE-2023-51765 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2023-51765 affect?

CVE-2023-51765 affects sendmail, freebsd, enterprise linux. Severity is medium (CVSS 5.3). sendmail through 8.17.2 allows SMTP smuggling in certain 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.

How urgent is CVE-2023-51765?

Priority 50/100. Public exploit reference; Remotely exploitable, no login, no user action; On your watchlist

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References

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