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A brief guide for CISO’s and DPO’s on email security in transit

Chief Technology Office Adam Low discusses the vulnerabilities in MTA-to-MTA communication over SMTP, opportunistic TLS, and how to email securely in his latest blog.

Email remains an essential communication channel for businesses worldwide. While most organizations are familiar with the standard email security trifecta — DKIM, DMARC, and SPF — there’s less awareness about the inherent risks in the transmission of emails between Mail Transfer Agents (MTAs). This guide focuses on the vulnerabilities in MTA-to-MTA communication over SMTP and highlights solutions like MTA-STS and DANE to secure emails in transit.

The Weak Links: SMTP Between MTAs

Simple graphical representation of MTA to MTA connection

SMTP, the bedrock of email communication, wasn’t initially designed with robust security in mind. As a result, emails transmitted between MTAs can be vulnerable to various threats like eavesdropping and man-in-the-middle attacks.

Over 330 billion emails are sent each day with several well known managed service providers (Google Mail, Microsoft Office 365, etc) accounting for the vast majority of the mailboxes originating and receiving email. Email is often used for the transmission of our most sensitive data and this statistic highlights the sheer volume of data we are sharing.

Opportunistic TLS: A Double-Edged Sword

Opportunistic TLS is the most widely deployed but is still left lacking

To counter the vulnerabilities in SMTP, Opportunistic TLS was introduced, automatically upgrading a plaintext connection to an encrypted one. While this is better than no encryption, Opportunistic TLS has multiple shortcomings:

  1. Downgrade Attacks: An attacker can interfere in the handshake process to force the connection to revert to plaintext.
  2. No Authentication: TLS alone doesn’t verify the identity of the receiving MTA, leaving room for impersonation.
  3. Certificate Vulnerabilities: Even if TLS is used, the certificates themselves can be a point of failure. Without the additional pinning that DANE provides, any compromise at the Certificate Authority level could lead to attackers obtaining legitimate certificates for malicious purposes.
  4. MITM is Real: The threat of Man-in-the-Middle (MITM) attacks is not theoretical; there have been instances of BGP hijacks rerouting IP traffic, allowing attackers to intercept or modify data in transit.

These issues demonstrate that Opportunistic TLS, while an advancement, falls short of providing comprehensive security for MTA-to-MTA communication.

Bridging the Gap: Advanced Technologies

We’ll explore these technologies in much more detail in a subsequent blog post, providing a clear comparison of which provides the greater levels of security.

MTA-STS (Mail Transfer Agent Strict Transport Security)

MTA-STS allows domain owners to publish policies that define how email sent to them should be securely transmitted. Adopting MTA-STS enables:

  1. Policy Enforcement: Mandatory usage of TLS for communication.
  2. Regular Checks: Routine validation of security configurations.

Google Mail has adopted MTA-STS but not DANE.

DANE (DNS-Based Authentication of Named Entities)

DANE uses DNSSEC to securely announce which TLS certificates a service should expect to receive when making a secure connection. By doing this, it effectively counters man-in-the-middle attacks.

Technically DANE is the most secure of the SMTP security standards available today. However many state cyber bodies recommend the use of MTA-STS, leaving no mention of DANE.

Microsoft are in the process of adopting DANE but at a relatively glacial pace.

The Awareness Problem: A Deeper Look

This is a conversation I’ve found myself having frequently and most recently with a fellow email security buff, Felix Mason. Why is awareness so low and why aren’t the large providers implementing these technologies?

Most of us are “blissfully” unaware of these weaknesses, once a message leaves our Outbox we really have no visibility of how securely it was transmitted to its destination.

The opportunistic nature of these improved security standards means CISO’s and DPO’s have little telemetry or insight into how secure email is being transmitted to third parties. Unfortunately I am only aware of one company (Zivver) that has developed the technology that enables the enforcement and assurance of minimum security standards for the transmission of email on to third party MTA’s.

Exclusive Choices: MTA-STS vs. DANE

Various email providers and even some governments have implemented either MTA-STS or DANE, often treating them as mutually exclusive options. This has created a fragmented landscape, reducing the universal efficacy of these technologies.

Lack of Awareness

The primary barrier to widespread adoption is arguably the lack of awareness about the weaknesses in MTA-to-MTA security. Many organizations operate under the assumption that existing measures like Opportunistic TLS are sufficient, which is far from the truth.

Call to Arms

If you’re in the process of procuring a new hosted mail service, it’s imperative to inquire about the provider’s methods to ensure secure email transit. Don’t just settle for Opportunistic TLS; ask for specifics about their implementation of MTA-STS, DANE, or both. Pushing for these technologies will not only bolster your organization’s security but also contribute to a more secure digital ecosystem.

Securing MTA-to-MTA email communication is critical but often overlooked. While Opportunistic TLS was a step in the right direction, it’s not sufficient for mature security postures. Solutions like MTA-STS and DANE offer more robust security but need to be more widely adopted. Increasing awareness and simplifying the implementation can drive more organizations to embrace these technologies.

Creating greater awareness and emphasis during procurement processes, should start to reduce the risk that many organisations may be unaware they have.

What next?

Do you know the difference between zero access encryption and end-to-end encryption? Read our latest blog to understand which is the best (and safest) solution for your organization.

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First published -
Last updated - 25/09/23
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