CVE-2023-51764

What it actually does

The vulnerability allows remote attackers to inject email messages with spoofed MAIL FROM addresses by exploiting SMTP smuggling via bare newline characters, bypassing SPF checks.

Why it matters to the business

Organizations may receive or relay spoofed emails that evade authentication mechanisms, increasing the risk of phishing attacks and damaging trust in email communications.

How an attack unfolds

An attacker crafts SMTP data with <LF>.<CR><LF> sequences to smuggle additional commands, injecting messages with forged sender addresses that other servers may not reject.

How to fix it

Configure smtpd_data_restrictions=reject_unauth_pipelining and smtpd_discard_ehlo_keywords=chunking, or enable smtpd_forbid_bare_newline=yes in versions 3.5.23, 3.6.13, 3.7.9, 3.8.4, or 3.9 and later.

How to confirm the fix worked

Test SMTP interactions to ensure bare newlines are rejected and check configuration for the specified restrictions.

Typical attack path

1. Identify a Postfix server without the recommended restrictions
2. Craft SMTP session with pipelined commands using bare newlines
3. Inject email with spoofed MAIL FROM
4. Bypass SPF validation on receiving servers

How to detect it

Review SMTP logs for unusual pipelining or bare newline usage in email transactions.

What your security team should do

• Apply the configuration changes immediately
• Upgrade Postfix to a supported version with the bare newline option
• Monitor for unusual email patterns