SMTP Smuggling: The New Technique Threatening Email Security by Exploiting Protocol Discrepancies

SMTP smuggling, a novel cybersecurity threat, has emerged as a significant concern due to its ability to exploit vulnerabilities in the Simple Mail Transfer Protocol (SMTP). This protocol is widely used by mail servers for the transmission, reception, and relaying of emails. Discovered by Timo Longin from SEC Consult, SMTP smuggling allows malicious actors to bypass established email authentication protocols and send spoofed emails, undermining the integrity and reliability of email communications.

Technical Overview: SMTP Smuggling

The vulnerability central to SMTP smuggling lies in the varying interpretations of the end-of-data sequence (“<CR><LF>.<CR><LF>”) among different SMTP servers. This sequence is critical in SMTP communications as it signifies the end of the email message content. SMTP, as a protocol, uses these specific character sequences to delineate different parts of an email. In this context, “<CR><LF>” represents the carriage return and line feed characters, which are standard text delimiters used to mark the end of a line in electronic text.

In a typical SMTP communication, when an email is sent, the end-of-data sequence signals to the server that the email body has concluded, and what follows should be treated as part of the SMTP protocol communication, rather than the email content. However, due to the inconsistent handling of this sequence across various SMTP implementations, attackers have found a way to exploit these inconsistencies to insert, or “smuggle,” additional SMTP commands into the email content.

Here’s a breakdown of how the exploitation works:

  1. Differing Interpretations: Some SMTP servers might interpret the end-of-data sequence in a non-standard way. For example, while one server might strictly adhere to the “<CR><LF>.<CR><LF>” sequence to denote the end of the message, another might accept just “<LF>.<LF>” as a valid end-of-data marker.
  2. Manipulating the End-of-Data Sequence: An attacker can craft an email message that includes what appears to be an end-of-data sequence, followed by additional SMTP commands. Due to the inconsistent interpretations, some servers will treat these additional commands as part of the email content, while others will execute them as SMTP commands.
  3. Spoofing and Bypassing Security Checks: By exploiting these discrepancies, attackers can manipulate the SMTP conversation to insert commands that spoof the sender’s email address or perform other malicious activities. This allows them to bypass security mechanisms like SPF, which are designed to validate the origin of email messages.
  4. Resulting in Spoofed Emails: The outcome is that emails can be sent that appear to originate from legitimate sources, but are actually crafted by attackers. These emails can bypass checks that would normally prevent spoofing, making them effective for phishing and other malicious activities.

SMTP smuggling, therefore, represents a significant security concern because it undermines the trust and reliability of email communications. The ability to bypass SPF and other email authentication mechanisms can lead to increased success in phishing attacks, where unsuspecting recipients may trust and act upon emails that appear to come from legitimate sources.

Affected Products and Services

This vulnerability predominantly impacts products from several key vendors, including Microsoft, GMX, and Cisco. Notably, Cisco Secure Email Gateway and Cisco Secure Email Cloud Gateway are among the affected products. The vulnerability is also present in open-source mail transfer agents like Postfix (CVE-2023-51764), Sendmail (CVE-2023-51765), and Exim (CVE-2023-51766).

Recommendations for Mitigation

To mitigate this risk, it is advised to change the default handling of carriage returns and line feed configurations in affected systems, particularly the Cisco Secure Email Cloud Gateway and Cisco Secure Email Gateway, to “Allow” instead of “Clean”. This simple yet critical adjustment prevents the exploitation of the vulnerability, enhancing the security of email communications.

In addition to these specific recommendations, organizations using affected SMTP servers should conduct thorough reviews of their email security protocols and configurations. Regular updates and patches provided by vendors should be applied promptly to address any emerging threats.


SMTP smuggling represents a significant challenge in the realm of email security, highlighting the ever-evolving nature of cyber threats. The ability of attackers to circumvent traditional security measures such as SPF, DKIM, and DMARC through this technique calls for a heightened level of vigilance and adaptive security measures. Organizations must stay informed about such vulnerabilities and take proactive steps to safeguard their email infrastructure against these sophisticated attack methods.

How Can Netizen Help?

Netizen ensures that security gets built-in and not bolted-on. Providing advanced solutions to protect critical IT infrastructure such as the popular “CISO-as-a-Service” wherein companies can leverage the expertise of executive-level cybersecurity professionals without having to bear the cost of employing them full time. 

We also offer compliance support, vulnerability assessments, penetration testing, and more security-related services for businesses of any size and type. 

Additionally, Netizen offers an automated and affordable assessment tool that continuously scans systems, websites, applications, and networks to uncover issues. Vulnerability data is then securely analyzed and presented through an easy-to-interpret dashboard to yield actionable risk and compliance information for audiences ranging from IT professionals to executive managers.

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