ClickCease SMTP Smuggling: Hackers Exploit New Flaw For Spoof Emails

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SMTP Smuggling: Hackers Exploit New Flaw For Spoof Emails

Wajahat Raja

January 17, 2024 - TuxCare expert team

In the ever-evolving landscape of cyber threats via email, a novel exploitation technique has emerged – Simple Mail Transfer Protocol – SMTP smuggling. This method, when wielded by threat actors, poses a significant risk by enabling the sending of spoof email threats with deceptive sender addresses, all while circumventing traditional security measures.


Malicious Use Of SMTP – A Brief Overview

SMTP, a fundamental TCP/IP protocol, serves as the backbone for sending and receiving email messages across networks. It establishes a connection between the email client and server to transmit email content. The server, facilitated by a mail transfer agent (MTA), verifies the recipient’s domain, ensuring the integrity of the email exchange.

The Anatomy of SMTP Smuggling

SMTP smuggling
hinges on the disparities in how outbound and inbound servers process end-of-data sequences. Exploiting these inconsistencies, threat actors can escape message data constraints, “smuggle” arbitrary SMTP commands, and even dispatch separate emails.

This nefarious technique draws inspiration from HTTP request smuggling, exploiting disparities in interpreting “Content-Length” and “Transfer-Encoding” HTTP headers. By inserting ambiguous requests into the inbound chain, attackers can compromise the integrity of email communications.

Threat Actors and SMTP Abuse


Major messaging servers, including those of Microsoft, GMX, and Cisco, have exhibited vulnerabilities to SMTP smuggling. These flaws allow threat actors to send malicious emails impersonating legitimate sources. Notably, Postfix and Sendmail, widely used SMTP implementations, are also susceptible.

Defeating Email Authentication Mechanisms

SMTP smuggling
enables threat actors to forge emails convincingly, appearing to originate from authentic sources. This poses a direct challenge to established authentication protocols such as DomainKeys Identified Mail (DKIM), Domain-based Message Authentication, Reporting, and Conformance (DMARC), and Sender Policy Framework (SPF).

Industry Response and Outlook

Promptly addressing the
SMTP protocol vulnerabilities, Microsoft and GMX have implemented fixes to counteract the potential exploitation of their messaging servers. However, the response from Cisco differs, asserting that the findings represent a “feature” rather than a vulnerability.

Persistent Risks

Despite rectifications by Microsoft and GMX, Cisco maintains that the identified issues are not vulnerabilities but features. Consequently, inbound SMTP smuggling to Cisco Secure Email instances remains feasible under default configurations. 

In protecting against SMTP attacks, users are advised by security experts to adjust their settings from “Clean” to “Allow” to mitigate the risk of receiving spoofed emails with valid DMARC checks.

Upcoming Changes in Email Security


Looking ahead, 2024 is poised to witness significant shifts in the realm of email security. Major providers like Google and Yahoo are set to implement more stringent requirements for sending emails through their services. These changes aim to bolster defenses against email spoofing techniques and spam, heralding a new era of heightened email security.

As we look into the future, it’s crucial to acknowledge the impending changes in email security. Major providers like Google and Yahoo are gearing up to enforce stricter requirements for email senders, signaling a collective effort to combat spam and spoofed emails. Staying informed and implementing email security best practices will be pivotal for businesses relying on these platforms for communication.



In the face of emerging threats like SMTP smuggling, organizations must stay vigilant and proactive. Understanding the intricacies of these email security vulnerabilities is the first step toward fortifying email security. As we anticipate industry-wide changes in 2024, aligning with evolving standards becomes imperative to ensure the resilience of our digital communication channels. 

Organizations must remain committed to automated patching solutions, contributing to a secure and resilient IT environment amid evolving cybersecurity risks in email communication.

The sources for this piece include articles in The Hacker News and ISP Today

SMTP Smuggling: Hackers Exploit New Flaw For Spoof Emails
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SMTP Smuggling: Hackers Exploit New Flaw For Spoof Emails
Explore the rising menace of SMTP smuggling and stay ahead in 2024. Learn how to safeguard against spoof emails. Your defense starts here!
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