What is Strict Transport Security (STS) for Email (MTA STS)?

SMTP MTA-STS (Mail Transfer Agent Strict Transport Security) is a security mechanism designed to enhance the security of email communication by preventing downgrade attacks and interception of email traffic. It is a protocol that allows email service providers to declare their support for encrypted email transmission and enforce the use of secure communication channels. Let’s break down the importance of an SMTP MTA-STS record and how it prevents downgrade attacks and interception.

any attacker who can delete parts of the SMTP session... can perform downgrade or interception attacks.

RFC 8471

• Preventing Downgrade Attacks: Downgrade attacks involve forcing a communication channel to use less secure protocols or encryption standards. In the context of email communication, attackers could attempt to manipulate the negotiation between email servers to use insecure transport mechanisms, like plain text SMTP, instead of encrypted ones, like STARTTLS. SMTP MTA-STS prevents such downgrade attacks by allowing email servers to declare their support for secure transport and requiring that communication between them always uses encryption.

• Enhancing Encryption: One of the primary goals of SMTP MTA-STS is to encourage the use of Transport Layer Security (TLS) encryption for email communication. TLS ensures that the communication between sending and receiving mail servers is encrypted and secure. Without proper encryption, email traffic can be intercepted and read by malicious actors or unauthorized entities.

• MTA-STS Policy and Records: An SMTP MTA-STS policy is established by the sender’s domain (the domain of the email sender). This policy is published as DNS records, specifically MTA-STS Policy and TLS Reporting, which are used by receiving mail servers to verify the sender’s stance on encryption and enforce secure communication.

• Preventing Man-in-the-Middle Attacks: Man-in-the-middle (MITM) attacks involve intercepting communication between two parties, often without either party’s knowledge. With SMTP MTA-STS, the published policy ensures that email servers only communicate with each other over encrypted channels. This makes it significantly more difficult for attackers to intercept and decipher email content.

SMTP MTA-STS plays a prominent role in bolstering the security of email communication by preventing downgrade attacks and interception. By enforcing the use of encrypted communication channels and requiring sender domains to publish their encryption policies, it helps ensure that email traffic remains confidential and protected from malicious actors.