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Security Vulnerability: SMTP Smuggling attack

This document (000021307) is provided subject to the disclaimer at the end of this document.


Any product running postfix / sendmail.


Security researchers have reviewed attacks against the SMTP protocol.

The attack could allow attackers to inject fake emails while bypassing some of the SMTP origin assurance methods like SPF. The attack relies on incorrect handling of the <CR><LF>.<CR><LF> sequence of the protocol of the SMTP data phase in some email servers.

While some hosted services are primary affected, also postfix and sendmail e-mail servers are affected and could receive smuggled emails.

Following CVEs were assigned:

CVE-2023-51764 for postfix
CVE-2023-51765 for sendmail
CVE-2023-51766 for exim


Postfix will provide an option to disable this bundling of e-mails:

    smtpd_forbid_unauth_pipelining = yes

Postfix updates with this option backported and default enabled will be released in the next days.
Solutions for sendmail will be provided once available.

Additional Information


This Support Knowledgebase provides a valuable tool for SUSE customers and parties interested in our products and solutions to acquire information, ideas and learn from one another. Materials are provided for informational, personal or non-commercial use within your organization and are presented "AS IS" WITHOUT WARRANTY OF ANY KIND.

  • Document ID:000021307
  • Creation Date: 22-Dec-2023
  • Modified Date:27-Dec-2023
    • SUSE Linux Enterprise Desktop
    • SUSE Linux Enterprise Server
    • SUSE Linux Enterprise Server for SAP Applications
    • SUSE Manager Server
    • SUSE Linux Enterprise Micro
    • SUSE Linux Enterprise HPC

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