CVE-2023-51764: Postfix through 3.8.5 allows SMTP smuggling unless configured with smtpd_data_restrictions=reject_unauth_pipel

This week

MEDIUM 5.3REMOTEWATCHLISTUPGRADEMITIGATEMITIGATE

CVE-2023-51764 is a medium-severity flaw (CVSS 5.3) affecting postfix, fedora, enterprise linux. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

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 p

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	2.6% (top 14%)
CISA exploited list	No
Weakness	CWE-345
Published	2023-12-24
Last modified	2026-10-08

Publisher (CNA): MITRE · Vendor: fedoraproject · postfix · redhat

Last updated 2026-10-08 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2023-12-24 05:15 UTCPublished CVE record published
2026-10-08 21:17 UTCUpdated Record last modified

postfix postfix UPGRADE

Affected: < 3.5.23 · >= 3.6.0 and < 3.6.13 · >= 3.7.0 and < 3.7.9 · >= 3.8.0 and < 3.8.4

Fixed in: 3.5.23, 3.6.13, 3.7.9, 3.8.4

Safe to downgrade to: a release before 3.6.0

1. Check installed version

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

2. UPGRADE to 3.5.23 (Debian / Ubuntu)

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

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade postfix

Note

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

3. If no update is available yet, DOWNGRADE to a release before 3.6.0 (Debian / Ubuntu)

apt-cache madison postfix sudo apt install postfix=<VERSION_BEFORE_3.6.0> sudo apt-mark hold postfix

3. If no update is available yet, DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list postfix sudo dnf downgrade postfix-<VERSION_BEFORE_3.6.0> sudo dnf versionlock add postfix

Downgrade risk

Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.

After downgrade

Restart the service, and keep the package held until a fixed release ships.

fedoraproject fedora MITIGATE

Affected: 38 · 39

Fixed in: no fixed release known

Safe to downgrade to: a release before 38

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-2023-51764?

postfix: upgrade to 3.5.23, 3.6.13, 3.7.9, 3.8.4; fedora: mitigate or apply the listed mitigation; enterprise linux: mitigate or apply the listed mitigation. Exact commands for each OS are on this page.

Is CVE-2023-51764 being exploited?

CVE-2023-51764 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-2023-51764 affect?

CVE-2023-51764 affects postfix, fedora, enterprise linux. Severity is medium (CVSS 5.3). 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

How urgent is CVE-2023-51764?

Priority 55/100. Public exploit reference; Remotely exploitable, no login, no user action; On your watchlist

Software pages

Postfix vulnerabilities

Related CVEs

CVE-2026-43964 LOW 3.7 UPGRADE Postfix before 3.8.16, 3.9 before 3.9.10, and 3.10 before 3.10.9 sometimes allows a buffer over-read and proce
CVE-2020-12063 MEDIUM 5.3 DOWNGRADE A certain Postfix 2.10.1-7 package could allow an attacker to send an email from an arbitrary-looking sender v
CVE-2017-10140 HIGH 7.8 UPGRADE Postfix before 2.11.10, 3.0.x before 3.0.10, 3.1.x before 3.1.6, and 3.2.x before 3.2.2 might allow local user
CVE-2012-0811 MEDIUM 6.5 UPGRADE Multiple SQL injection vulnerabilities in Postfix Admin (aka postfixadmin) before 2.3.5 allow remote authentic
CVE-2011-1720 MEDIUM 6.8 DOWNGRADE The SMTP server in Postfix before 2.5.13, 2.6.x before 2.6.10, 2.7.x before 2.7.4, and 2.8.x before 2.8.3, whe

References

← Back to latest CVEs