Exchange internal Send connectors must use domain security (mutual authentication Transport Layer Security).

Overview

Finding ID	Version	Rule ID	IA Controls	Severity
V-221218	EX16-ED-000170	SV-221218r811174_rule		Medium

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 authentication method used for communications between servers. With this feature enabled, only servers capable of supporting domain authentication 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.

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 authentication method used for communications between servers. With this feature enabled, only servers capable of supporting domain authentication 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-22933r811172_chk )
Open the Exchange Management Shell and enter the following command: Get-SendConnector \| Select Name, Identity, DomainSecureEnabled, DNSRoutingEnabled, RequireTLS, TlsAuthLevel For each send connector: If the send connector does not use a smarthost and the value of "DomainSecureEnabled" is not set to "True", this is a finding. If the send connector does use a smarthost, "DomainSecureEnabled" will be set to "False". If the send connector does use a smarthost "DNSRoutingEnabled", "RequireTLS", and "TlsAuthLevel" must be set. If the send connector using a smart host has a value for “DNSRoutingEnabled” that is not set to “False”, this is a finding. 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.

Check Text ( C-22933r811172_chk )

Open the Exchange Management Shell and enter the following command:

Get-SendConnector | Select Name, Identity, DomainSecureEnabled, DNSRoutingEnabled, RequireTLS, TlsAuthLevel

For each send connector:

If the send connector does not use a smarthost and the value of "DomainSecureEnabled" is not set to "True", this is a finding.

If the send connector does use a smarthost, "DomainSecureEnabled" will be set to "False".

If the send connector does use a smarthost "DNSRoutingEnabled", "RequireTLS", and "TlsAuthLevel" must be set.

If the send connector using a smart host has a value for “DNSRoutingEnabled” that is not set to “False”, this is a finding.

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-22922r811173_fix)
Open the Exchange Management Shell and enter the following command: Set-SendConnector <'IdentityName'> -DomainSecureEnabled $true Note: The value must be in single quotes. Repeat the procedures for each send connector. The following commands can be executed if smarthosts are used: Set-SendConnector <'IdentityName'> -RequireTLS $true -DNSRoutingEnabled $False -TlsAuthLevel $DomainValidation