Exchange nonexistent recipients must not be blocked.

Overview

Finding ID	Version	Rule ID	IA Controls	Severity
V-69869	EX13-EG-000185	SV-84491r1_rule		Medium

Description
Spam originators, in an effort to refine mailing lists, sometimes use a technique where they first create fictitious names and then monitor rejected emails for non-existent recipients. Those not rejected are deemed to exist and are used in future spam mailings. To prevent this disclosure of existing email accounts to spammers, email to nonexistent recipients must not be blocked. Instead, it is recommended that all messages be received, then evaluated and disposed of without enabling the sender to determine existent vs. nonexistent recipients.

Description

Spam originators, in an effort to refine mailing lists, sometimes use a technique where they first create fictitious names and then monitor rejected emails for non-existent recipients. Those not rejected are deemed to exist and are used in future spam mailings. To prevent this disclosure of existing email accounts to spammers, email to nonexistent recipients must not be blocked. Instead, it is recommended that all messages be received, then evaluated and disposed of without enabling the sender to determine existent vs. nonexistent recipients.

STIG	Date
MS Exchange 2013 Edge Transport Server Security Technical Implementation Guide	2018-09-18

Details

Check Text ( C-70337r1_chk )
Open the Exchange Management Shell and enter the following command: Get-RecipientFilterConfig \| Select Name, RecipientValidationEnabled If the value of RecipientValidationEnabled is not set to False, this is a finding.

Fix Text (F-76099r1_fix)
Open the Exchange Management Shell and enter the following command: Set-RecipientFilterConfig -RecipientValidationEnabled $false