Microsoft is working to add SMTP MTA Strict Transport Security (MTA-STS) support to Exchange Online to ensure the security and integrity of Office 365 email clients.
Once MTA-STS is available in Office 365 Exchange Online, emails sent by users via Exchange Online will only be delivered via authentication and encryption links, protecting against both email spying and attacks. .
Protection against MITM and downgrade attacks
MTA-STS enhances the security of Exchange Online email and solves many SMTP security problems, such as lack of support for secure protocols, TLS certificates which have expired and certificates not issued by trusted third parties or corresponding server domain names.
As the mail servers will continue to send emails, although it is not possible to creation a secure TLS connection, SMTP connections are exposed to various attacks, such as downgrade attacks and Man-in-the-middle.
"Downgrade attacks are possible where the STARTTLS response can be deleted, thus delivering the message in clear text," says Microsoft. "Man-in-the-middle (MITM) attacks are also possible, with which the message can be redirected to a server intruder. "
"MTA-STS (RFC8461) helps prevent such attacks providing a mechanism for determining domain policies that determine whether the received domain supports TLS and what to do when TLS cannot be traded, ”the company explains in an update to Microsoft 365.
"Outgoing Exchange Online Mail (EXO) now supports MTA-STS," Microsoft added.
Exchange Online SMTP MTA Strict Transport Security (MTA-STS) support is in progress and company plans to make it generally available in December in all environments, for all Exchange Online users.