What is SMTP smuggling?

SMTP smuggling is a new technique that exploits subtle inconsistencies in how different email servers (Mail Transfer Agents or MTAs) interpret the end-of-data sequence in the SMTP protocol. This allows attackers to inject (smuggle) unauthorized email messages that appear legitimate, bypassing standard security checks.

How does SMTP smuggling bypass SPF, DKIM, and DMARC?

The smuggled emails bypass SPF, DKIM, and DMARC because they are injected into an existing, legitimate SMTP session initiated by a vulnerable sending server. The receiving server perceives the smuggled email as part of the authorized communication from a valid source, causing it to pass authentication checks that would normally fail for spoofed emails.

What are the real-world risks associated with SMTP smuggling?

The primary risks include highly effective phishing attacks, Business Email Compromise (BEC) scams, and brand impersonation. Since the emails appear legitimate and pass authentication, recipients are more likely to trust them, leading to potential data breaches, financial fraud, and significant reputational damage for impersonated brands.

How can organizations protect themselves against SMTP smuggling?

Organizations can protect themselves by ensuring their Mail Transfer Agents (MTAs) are fully patched with updates that address SMTP smuggling vulnerabilities. Implementing and enforcing strict DMARC policies (p=quarantine or p=reject) is crucial, along with continuous monitoring of DMARC reports for any unusual activity or anomalies that might indicate a smuggling attempt. User awareness training on advanced phishing techniques is also vital.

Is SMTP smuggling a protocol flaw or an implementation flaw?

SMTP smuggling is generally considered an implementation flaw rather than a flaw in the SMTP protocol itself. The protocol's specifications allow for some interpretation, and it is these differing interpretations between various mail server software that attackers exploit. Vendors like Microsoft have already released patches to address these specific implementation vulnerabilities.

What are the implications of the new SMTP smuggling technique? - Technical - Email deliverability - Knowledge base

The discovery of the new SMTP smuggling technique has created significant concern across the email security landscape. This novel vulnerability allows malicious actors to send forged emails that appear legitimate, even bypassing robust authentication protocols like SPF, DKIM, and DMARC. It represents a subtle yet impactful threat to the integrity of email communication, underscoring the continuous evolution of cyber threats.

At its core, SMTP smuggling exploits discrepancies in how different mail transfer agents (MTAs) interpret the Simple Mail Transfer Protocol (SMTP), particularly regarding the end-of-data sequence. When an attacker can manipulate these inconsistencies, they can inject malicious content or commands that are processed differently by the sending and receiving servers, effectively "smuggling" unauthorized emails past defenses.

The implications are far-reaching for businesses, email service providers, and individual users, as it can enable highly convincing phishing attacks and widespread brand impersonation. Understanding how this technique works and its potential impacts is crucial for anyone involved in email operations or cybersecurity.

How SMTP smuggling works

SMTP smuggling leverages a fundamental ambiguity in how the end-of-data sequence for an email message is handled. The SMTP protocol uses a specific sequence, a dot on a line by itself, to signify the end of the message body. However, some mail servers are more lenient in their interpretation of this sequence than others.

The attack occurs when a sender uses an SMTP client that sends a malformed end-of-data sequence. A vulnerable sending mail server might interpret this sequence prematurely, believing the message has ended, while a more permissive receiving mail server continues to process subsequent data as part of the original email. This creates a window for an attacker to "smuggle" additional SMTP commands and an entirely new email message within the perceived same session. Researchers from Usenix Security first detailed this technique, highlighting its potential to undermine email integrity.

The smuggled email, sent by the attacker, appears to originate from the sending server and can then bypass security checks that rely on the initial SMTP conversation. This exploitation of differing interpretations between MTA implementations, rather than a flaw in the SMTP protocol itself, is what makes the technique so insidious.

The bypass of email authentication

Perhaps the most alarming implication of SMTP smuggling is its ability to bypass established email authentication protocols. These protocols, including Sender Policy Framework (SPF), DomainKeys Identified Mail (DKIM), and Domain-based Message Authentication, Reporting & Conformance (DMARC), are designed to verify the legitimacy of email senders and prevent spoofing.

Because the smuggled email appears to come from the compromised sending server, it effectively inherits the server's legitimate authentication. This means an email originating from an attacker can pass SPF, DKIM, and DMARC checks, making it incredibly difficult for recipient mail servers to distinguish it from genuine correspondence. This is a significant challenge for existing defenses, as traditional email authentication relies on consistent protocol interpretation.

This vulnerability is particularly concerning when considering how easily a phishing email can pass SPF and DKIM checks. It highlights a critical blind spot for many organizations that heavily rely on these protocols for their email security posture. If your organization is enforcing DMARC policies, it's important to understand this new attack vector.

The DMARC challenge

DMARC works by instructing receiving mail servers how to handle emails that fail SPF or DKIM. However, in an SMTP smuggling scenario, the forged email passes these checks because it originates from a server that is otherwise legitimately authorized. This means even a strong DMARC policy (p=quarantine or p=reject) may not catch smuggled emails, as they technically 'pass' authentication.

This necessitates a re-evaluation of how DMARC reports are interpreted and how email security solutions can detect subtle anomalies that might indicate smuggling attempts, even when standard authentication appears to pass. It's crucial to look beyond simple pass/fail results for deeper insights.

Broader implications for email security

The real-world implications of SMTP smuggling are severe, primarily escalating the threat of phishing and Business Email Compromise (BEC) attacks. Attackers can now send highly convincing spoofed emails that appear to come from legitimate domains, including those of major companies like

Microsoft and

Amazon. This makes it significantly harder for users to identify malicious emails, increasing the likelihood of successful social engineering.

For organizations, the risk of brand impersonation is heightened. If your domain is exploited, your customers and partners could receive phishing emails seemingly from you, leading to reputational damage, financial loss, and erosion of trust. Furthermore, these attacks can result in your legitimate emails being blocklisted or blacklisted, impacting your email deliverability rates.

The vulnerability highlights the ongoing need for robust security measures beyond standard authentication, including advanced threat detection, user awareness training, and continuous monitoring of email traffic. The original research from SEC Consult underscored how widespread the issue was, affecting numerous major email providers.

Mitigating the threat

Mitigating SMTP smuggling requires a multi-faceted approach, involving both senders and receivers. For sending organizations, it's critical to ensure that your mail servers are properly configured to adhere strictly to SMTP standards, preventing the premature termination of message data. This includes applying patches and updates from your MTA vendor, such as

Postfix or

Sendmail, to address known vulnerabilities.

Receiving mail servers should implement stricter parsing of SMTP data, being less tolerant of malformed sequences that could indicate a smuggling attempt. This means going beyond basic SPF and DKIM checks and implementing advanced anomaly detection. For a deeper dive into improving your overall deliverability, consider exploring our guide on technical solutions for top performing senders.

Organizations should also regularly review their DMARC reports for unusual patterns, even if authentication appears to pass. This includes examining details beyond simple pass/fail flags to identify subtle inconsistencies. Proactive measures, such as tightening DMARC policies to quarantine or reject emails that fail alignment, remain critical.

For senders

Patch MTAs: Apply all available patches from mail server software vendors immediately to address SMTP protocol interpretation issues.
Enforce strict DMARC: Gradually move your DMARC policy to p=quarantine or p=reject to minimize the impact of spoofed emails.
Monitor DMARC reports: Regularly analyze aggregated and forensic DMARC reports for anomalies that might indicate smuggling attempts.

For receivers

Stricter SMTP parsing: Configure receiving mail servers to enforce strict adherence to SMTP protocol, preventing premature end-of-data interpretations.
Advanced threat detection: Implement email security solutions with advanced capabilities to detect anomalies that bypass standard authentication.
User training: Educate users on identifying phishing attempts, even if emails appear to pass authentication checks.

Views from the trenches

Best practices

Always apply the latest patches and updates to your MTA software to address known SMTP vulnerabilities and interpretation discrepancies.

Implement a strict DMARC policy (p=reject) for your domain as soon as possible, after careful monitoring, to prevent spoofing.

Continuously monitor your DMARC reports for unexpected email sources or authentication anomalies, even for passing emails.

Educate your users on advanced phishing tactics and how to identify suspicious emails, regardless of authentication status.

Review your email sending infrastructure for any non-standard SMTP behaviors that could inadvertently contribute to smuggling risks.

Common pitfalls

Relying solely on SPF and DKIM for email authentication, as SMTP smuggling can bypass these mechanisms effectively.

Ignoring DMARC reports or failing to move beyond a 'p=none' policy, which leaves your domain vulnerable to impersonation.

Neglecting to apply timely patches to mail servers, leaving critical vulnerabilities unaddressed for attackers to exploit.

Assuming that all emails passing authentication are legitimate, without deeper analysis for subtle signs of compromise.

Having overly permissive configurations on receiving mail servers that do not strictly enforce SMTP protocol standards.

Expert tips

Consider deploying an email security gateway that performs deep packet inspection and anomaly detection beyond standard authentication.

Implement multi-factor authentication for all email accounts to prevent unauthorized access, which complements technical email security.

Regularly conduct phishing simulations to test your employees' vigilance against sophisticated attacks, including those enabled by smuggling.

Collaborate with your email service providers to understand their stance and protective measures against SMTP smuggling and similar threats.

Automate the analysis of DMARC forensic reports to quickly identify and respond to any potential smuggling incidents targeting your domain.

Expert view

Expert from Email Geeks says that SMTP smuggling is an implementation issue rather than a protocol issue. It's expected that named vendors will correct it quickly.

December 20, 2023 - Email Geeks

Expert view

Expert from Email Geeks says it will be interesting to see how many legitimate emails fail when stricter compliance with the SMTP specification is enforced.

December 20, 2023 - Email Geeks

Maintaining email integrity in a changing threat landscape

The SMTP smuggling technique serves as a stark reminder that email security is an ever-evolving field. While protocols like SPF, DKIM, and DMARC are robust, their effectiveness relies on consistent implementation across the complex ecosystem of mail servers. Organizations must remain vigilant, prioritize software updates, and adopt comprehensive security strategies that go beyond basic authentication checks to safeguard their email communications and maintain their email domain reputation.