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Exchange internal Send connectors must require encryption.


Overview

Finding ID Version Rule ID IA Controls Severity
V-221262 EX16-ED-000690 SV-221262r811179_rule High
Description
The Simple Mail Transfer Protocol (SMTP) connector is used by Exchange to send and receive messages from server to server. Several controls work together to provide security between internal servers. This setting controls the encryption method used for communications between servers. With this feature enabled, only servers capable of supporting Transport Layer Security (TLS) will be able to send and receive mail within the domain. The use of secure communication prevents eavesdroppers from reading or modifying communications between mail clients and servers. While sensitive message bodies should be encrypted by the sender at the client, requiring a secure connection from server to server adds protection by encrypting the sender and recipient information that cannot be encrypted by the sender. Individually, channel security and encryption can be compromised by attackers. Used together, email becomes a more difficult target, and security is heightened. Failure to enable this feature gives eavesdroppers an opportunity to read or modify messages between servers.
STIG Date
Microsoft Exchange 2016 Edge Transport Server Security Technical Implementation Guide 2022-06-09

Details

Check Text ( C-22977r811177_chk )
Open the Exchange Management Shell and enter the following command:

Get-SendConnector | Select Name, Identity, TlsDomain

If the value of "TlsDomain" is not set to the value of the internal <'SMTP Domain'>, this is a finding.

Get-SendConnector | Select Name, Identity, DomainSecureEnabled

If the value of "DomainSecureEnabled" is not set to "True" and the SendConnector is not using a smarthost, this is a finding.

If the value of "DomainSecureEnabled" is set to "False" and the send connector is using a smarthost, the value of “RequireTLS” must be set to “True”, and the value for “TlsAuthLevel” must be set to “DomainValidation”.

If the send connector using a smarthost has a value for “RequireTLS” that is not set to “True”, this is a finding.

If the send connector using a smarthost has a value for “TlsAuthLevel” that is not set to “DomainValidation”, this is a Finding.
Fix Text (F-22966r811178_fix)
Open the Exchange Management Shell and enter the following command:

Set-SendConnector -Identity <'IdentityName'> -TlsDomain <'SMTP Domain'>

Set-SendConnector -Identity <'ReceiveConnector'> -DomainSecureEnabled 'True'

Note: The SMTP Domain is the internal SMTP domain within the organization.

The following commands can be executed if smarthosts are used:

Set-SendConnector <'IdentityName'> -RequireTLS $true -TlsAuthLevel $DomainValidation