Microsoft is working on adding SMTP MTA Strict Transport Security (MTA-STS) support to Exchange Online to ensure Office 365 customers’ email communication security and integrity.
Once MTA-STS is available in Office 365 Exchange Online, emails sent by users via Exchange Online will only one delivered using connections with both authentication and encryption, protecting against both email interception and attacks.
Protection against MITM and downgrade attacks
MTA-STS strengthens Exchange Online email security and solves multiple SMTP security problems including the lack of support for secure protocols, expired TLS certificates, and certs not issued by trusted third parties or matching server domain names.
Given that mail servers will still deliver emails even though a properly secured TLS connection can’t be created, SMTP connections are exposed to various attacks including downgrade and man-in-the-middle attacks.
“[D]owngrade attacks are possible where the STARTTLS response can be deleted, thus rendering the message in clear text,” Microsoft says. “Man-in-the-middle (MITM) attacks are also possible, whereby the message can be rerouted to an attacker’s server.”
“MTA-STS (RFC8461) helps thwart such attacks by providing a mechanism for setting domain policies that specify whether the receiving domain supports TLS and what to do when TLS can’t be negotiated, for example stop the transmission,” the company explains in a Microsoft 365 roadmap entry.
“Exchange Online (EXO) outbound mail flow now supports MTA-STS,” Microsoft also adds.
Exchange Online SMTP MTA Strict Transport Security (MTA-STS) support is currently in development and the company is planning to make it generally available during December in all environments, for all Exchange Online users.
DNSSEC and DANE for SMTP also coming
Microsoft is also working on including support for the DNSSEC (Domain Name System Security Extensions) and DANE for SMTP (DNS-based Authentication of Named Entities) to Office 365 Exchange Online.
Support for the two SMTP standards will be added to both inbound and outbound mail, “specific to SMTP traffic between SMTP gateways” according to the Microsoft 365 roadmap [1, 2] and this blog post.
According to Microsoft, after including support for the two SMTP security standards in Exchange Online:
DANE for SMTP will provide a more secure method for email transport. DANE uses the presence of DNS TLSA resource records to securely signal TLS support to ensure sending servers can successfully authenticate legitimate receiving email servers. This makes the secure connection resistant to downgrade and MITM attacks.
DNSSEC works by digitally signing records for DNS lookup using public key cryptography. This ensures that the received DNS records have not been tampered with and are authentic.
Microsoft is planning to release DANE and DNSSEC for SMTP in two phases, with the first one to include only outbound support during December 2020 and with the second to add inbound support by the end of next year.