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Exchange internal receive connectors must not allow anonymous connections.


Finding ID Version Rule ID IA Controls Severity
V-259687 EX19-MB-000135 SV-259687r942375_rule Medium
This control is used to limit the servers that may use this server as a relay. If a Simple Mail Transport Protocol (SMTP) sender does not have a direct connection to the internet (for example, an application that produces reports to be emailed), it will need to use an SMTP Receive connector that does have a path to the internet (for example, a local email server) as a relay. SMTP relay functions must be protected so third parties are not able to hijack a relay service for their own purposes. Most commonly, hijacking of relays is done by spammers to disguise the source of their messages and may also be used to cover the source of more destructive attacks. Relays can be restricted in one of three ways: by blocking relays (restrict to a blank list of servers), by restricting use to lists of valid servers, or by restricting use to servers that can authenticate. Because authenticated connections are the most secure for SMTP Receive connectors, it is recommended that relays allow only servers that can authenticate.
Microsoft Exchange 2019 Mailbox Server Security Technical Implementation Guide 2024-01-10


Check Text ( C-63426r942373_chk )
Open the Exchange Management Shell and enter the following command:

Get-ReceiveConnector | Select-Object -Property Name, Identity, PermissionGroups |Format-List

For each Receive connector, if the value of "PermissionGroups" is "AnonymousUsers" for any receive connector, this is a finding.
Fix Text (F-63334r942374_fix)
Open the Exchange Management Shell and enter the following command:

Set-ReceiveConnector -Identity ['IdentityName'] -PermissionGroups and enter a valid value user group.

Note: The value must be in quotes.

Example: Set-ReceiveConnector -Identity <'IdentityName'> -PermissionGroups ExchangeUsers

Repeat the procedures for each Receive connector. This will remove the AnonymousUsers value simultaneously.