4.6 Ensure the Client Rules Forwarding Block is enabled

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This audit has been deprecated and will be removed in a future update.

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Information

You should enable the Client Rules Forwarding Block, which prevents the use of any client-side rules that forward email to an external domain.

Rationale:

The use of client-side forwarding rules to exfiltrate data to external recipients is an increasingly used vector for data exfiltration by bad actors.

Impact:

Care should be taken before implementation to ensure there is no business need for case-by-case auto-forwarding. Disabling auto-forwarding to remote domains will affect all users and in an organization.

NOTE: Nessus has not performed this check. Please review the benchmark to ensure target compliance.

Solution

To create the Client Rules Forwarding Block, use the Exchange Online PowerShell Module:

Connect to Exchange Online using Connect-ExchangeOnline.

Run the following Exchange Online PowerShell commands to create a rule:

$rejectMessageText = 'To improve security, auto-forwarding rules to external addresses have been disabled. Please contact your Microsoft Partner if you'd like to set up an exception.'

New-TransportRule -name 'Client Rules To External Block' -Priority 0 -SentToScope NotInOrganization -FromScope InOrganization -MessageTypeMatches AutoForward -RejectMessageEnhancedStatusCode 5.7.1 -RejectMessageReasonText $rejectMessageText

Verify that Client Rules To External Block gets created.

See Also

https://workbench.cisecurity.org/files/3433