VulnerabilityPublished 24 Dec 2023Updated 4 Nov 2025

CVE-2023-51764: postfix · CVE-2023-51764 is a

Affects postfixSource CVE Database V5

Not known to be exploited.

Data as of 26 Oct 2026, 13:47 UTC

Severity

Medium

Rated Medium by the CNA (mitre) · No CVSS score from NVD or the CNA

EPSS: 2.6% chance of exploitation in 30 days – higher than 85% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.

Exploitation evidence

Disclosed: evidence found.
CVE published 24 Dec 2023
Detectable: no evidence on record.
No public nuclei template indexed
Weaponised: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
Exploited in the wildExploited: no evidence on record.
Not in CISA KEV and no source reports exploitation
Observed by OffSeq MirageSeen by Mirage: no evidence on record.
No attack traffic recorded by OffSeq Mirage sensors

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 26 Oct 2026.

Disclosed: The CVE record is public on cve.org. Its publication is t = 0.
Detectable: A public nuclei template can find exposed instances, for defenders and attackers alike.
Weaponised: Public exploit code exists in Metasploit or Exploit-DB, or CISA SSVC or the CVSS exploit maturity reports a public proof of concept.
Exploited in the wild: CISA KEV lists it, CISA SSVC rates exploitation active, or a source reports attacks.
Observed by OffSeq Mirage: OffSeq Mirage honeypot sensors recorded attack traffic for this CVE, or found exposed hosts without attack traffic.

Moments are spaced evenly, not to scale; a dashed line marks a gap of six months or more. Deltas count from t = 0. Sources that give only a date (CISA KEV, Exploit-DB, or a time of exactly 00:00Z) read “same day” rather than hours. Metasploit modules carry the flaw’s disclosure date, not their release date, and nuclei templates carry no date, so both appear above but not on the timeline.

What to do

Affected software

Product	Affected	Fixed in
Debianpostfix`pkg:deb/postfix`Other threats	Affected <=3.8.5	Fixed in–

Check your own hosts for these packages with threat-finder (opens in a new tab), our free open-source scanner.

AI-suggested

Partial mitigation is possible by configuring Postfix with smtpd_data_restrictions=reject_unauth_pipelining and smtpd_discard_ehlo_keywords=chunking, which blocks misuse of SMTP command pipelining but does not fully address message pipelining or malformed line endings. The only complete remediation is to upgrade Postfix to versions 3.5.23, 3.6.13, 3.7.9, 3.8.4, 3.9 or later and enable smtpd_forbid_bare_newline=yes. Users should follow vendor advisories and apply package updates when available.

Description

CVE-2023-51764 is a vulnerability in Postfix versions up to 3.8.5 that allows SMTP smuggling due to default configurations accommodating older SMTP clients. This flaw enables remote attackers to inject email messages with spoofed MAIL FROM addresses, potentially bypassing SPF protections. The issue arises because Postfix supports a non-standard line ending sequence (<LF>.<CR><LF>) that some other mail servers do not. Partial mitigations exist via configuration options, but a complete fix requires package updates to versions including the smtpd_forbid_bare_newline option.

Impact

Remote attackers can exploit this vulnerability to inject email messages with spoofed MAIL FROM addresses, potentially bypassing SPF protection mechanisms. This can undermine email authentication and increase the risk of phishing or spam. The impact is considered moderate due to the requirement for specific server configurations and conditions for successful exploitation.

Summary

Postfix versions up to 3.8.5 are vulnerable to SMTP smuggling attacks that exploit the server's support for <LF>.<CR><LF> line endings, allowing attackers to inject spoofed emails that can bypass SPF checks. This vulnerability is due to default configurations that permit SMTP command pipelining and do not forbid bare newlines, which some other SMTP servers reject. Partial mitigation can be achieved by setting smtpd_data_restrictions=reject_unauth_pipelining and smtpd_discard_ehlo_keywords=chunking, but these do not fully address message pipelining or malformed line endings. The definitive fix involves upgrading to Postfix versions 3.5.23, 3.6.13, 3.7.9, 3.8.4, or 3.9 or later, which support the smtpd_forbid_bare_newline=yes option to prevent such attacks.

References

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